Crannóg, The Island Lake Dwelling Of Celtic And Medieval Ireland

In simple terms, a crannóg is an artificial island of variable size and height, roughly circular or oval in shape, constructed on the bed of a lake or on a suitable mudbank or islet. While some examples were surrounded by waters shallow enough to wade through it was more usual for the semi-submerged mounds to be connected by a man-made causeway or footbridge to dry land or to make use of boats and canoes for access. Though some crannóga were permanently inhabited, others were occupied intermittently, occasionally lying abandoned for decades at a time. In general, these types of lake-dwellings seem to have been an almost uniquely Gaelic feature. That is, Irish and Scottish. They were in use from the Late Bronze Age to the pre-industrial era, a span of some three thousand years. However the most intensive periods of building seems to have lain between 800 BCE and 200 CE in Scotland, and 400 CE to 1100 CE in Ireland. The late Irish dates coincided with significant cultural, social, political and technological changes in the country though these do not adequately explain the broader motivation for building artificial lake-habitations or their enduring popularity over the millennia.

As for the modern name, crannóg is derived from the Irish words crann “tree” and óg “young”. It is first encountered in the late 12th century, applied to the artificial island and to the structures on it, though it is not always clear which one is being referred to. However far earlier records describe the lake monuments as an inis or oileán, both translating as “island”. Occasionally these descriptions might be prefaced by the term “fortified” but by and large the Medieval Irish did not distinguish between natural and man-made isles, seeing them as similar features in the landscape.

The location and construction of a crannóg

Most crannóga were situated on small lakes, usually one per location, though slightly bigger bodies of open water may have contained several artificial islands of lesser or greater sizes (in some cases these habitats were clustered together; in others they were spaced well apart). Large lakes were avoided, probably to minimise the danger posed by strong waves and currents during storms which may have damaged or overwhelmed the exposed platforms. More often than not the crannóg-builders looked to natural inlets or coves along the shore to provide some degree of shelter, the relatively low waters in these locations aiding construction or providing ready access to the artificial mounds when complete. Where possible, existing features in the lake, surface shoals or banks, were used to anchor the crannóg, reducing the material and labour costs. In contrast only a comparatively small number of platforms were sited in deep water, sacrificing convenience for the security that physical distance provided.

In many cases the islands were built in locations of regional or local significance. While a few stood relatively isolated, most shared the landscape with far older monuments such as burial mounds or cairns, possibly promoting notions of continuity with past generations. This was important in a largely pastoral narrowly resourced society where disputes over land rights were not unknown. A fresh-water lake and the fertile pastures around it would have been a valuble resource desired by many. Some crannóga, even those rarely occupied, may have functioned as physical declarations of ownership for families or communities whose livelihoods depended on the well-being of their cattle and other livestock, rather than on cereal production.

Navigable rivers and water-crossings, which served both as borders and routes of communication between different tuatha or “kingdoms”, would have been another factor influencing location. By exploiting the trade which moved along these lines, by boat or by foot, some platforms could have enriched themselves or become centres for bartering and exchange. It is clear that in many localities the contemporary relevance of the area may well have decided for or against the presence of a crannóg, though the exact thinking of the builders and inhabitants can only be guessed at.

An artist's impression of a typical-sized crannóg in a small lake, close to the shore. While the majority of crannóga conformed to this template some would have been far larger and more elaborate in nature
An artist’s impression of a typical-sized crannóg in a small lake, close to the shore. While the majority of crannóga conformed to this template some would have been far larger and more elaborate in nature

Whatever the initial incentive, the construction of a crannóg required considerable community effort, probably from the late spring to the early autumn when most bodies of open water were calmer and in some cases slightly lower. To begin with, a large number of upright wooden posts, sharpened at one end, were lowered into the lake and hammered deep into the mud below. These vertical piles sometimes used interlocking joints or other architectural techniques to improve their rigidity. This resulted in a more-or-less circular enclosure of tightly spaced poles in the water, the heads visible above the surface. A plan of this shape is rather easy to achieve, which may be one of the reasons why the Medieval Irish favoured it so much in their homes and settlements. A circle can be created by simply placing one end of a predetermined length of rope at a central point, held by a kneeling person or tied to a short stick, and pulling tightly on the opposite end as you move around the fixed centre. This will create a relatively even and equidistant circle. Since most crannóga were originally fairly uniform in shape some similar technique must have been used by their builders, albeit with far greater difficulty given the environment.

Depending on local resources or requirements, the planned lake-dwelling could range from four or five metres in diameter to over thirty-five. The interior area was filled with alternate layers of rock, peat and broken brushwood, the order of stratification changing from site to site. It’s likely that heavy stones and human feet were used to laboriously tamp down the layers, hopefully removing dangerous voids or cavities which could collapse the interior of the structure, as well as driving out pooled water. After months of toiling the artificial mound would rise from the lake, the height determined by the builders. When the platform was judged to be sufficiently stable, the top would be levelled off and finished with a compacted mixture of soil, sand and pebbles to provide a solid surface and some degree of drainage. As an added advantage, this also gave a non-combustible floor to live upon. One of the great ironies of the crannóga must have been their vulnerability to fire. In the event of a serious conflagration, accidental or deliberate, the islands would have been more of a trap than a haven.

This labour undoubtedly involved considerable effort and hardship on the part of the community as men (and maybe women) worked in water and mud for several days or weeks at a time, perhaps across the space of half a year or more. While some materials could have been floated to the location, others would have required multiple raft- or boat-trips from the shore, not to mention being brought to the lakefront in the first place by foot or cart. The work was no doubt dangerous too as the builders risked injury, illness or drowning in the difficult conditions. Though prehistoric populations in Ireland and Scotland were capable of incredible feats of construction, other activities, from farming to hunting, child-rearing to cooking, still required their daily attention in order to survive.

It’s likely that some of the new islands may have needed a second or third round of layering as they settled into the soft lakebed, unexpected problems arising even after completion. Likewise, the first rains or storms might well have exposed any other flaws in the building work, perhaps requiring further remedial labour. The addition of plank or flagstone revetments around the wave-lapped circumference of the crannóg was apparently deemed necessary in a number of cases, though this might have been for cosmetic rather than architectural reasons. On a larger timescale natural or man-made wear and tear made some form of substantial refurbishment inevitable, additional construction layers raising the platforms higher and outward. This mixture of permanence and impermanence gave crannóga a protean aspect which their inhabitants were well aware of.

In terms of access, while narrow bridges of timber planking or wattle may have existed, most islands seem to have relied on stone and turf causeways for entry. Some of these were partly submerged, possibly for reasons of concealment, requiring ankle or knee high wading. In a number of locations the paths followed circuitous routes across the water rather than leading directly from the shore, possibly to make visitors more visible to the island’s inhabitants or to create a grander impression upon arrival at the crannóg proper. The latter intention was certainly obvious in the case of the larger lake-platforms, quite a few of which were surrounded by timber and stone kerbing at the water’s edge, with high palisades above (from the 6th or 7th century some of these may have been painted with white lime, a relatively late decorative practice in Ireland). There is some evidence that these types of prestige crannóga were the seasonal or regular abodes of leading lords and kings, or of important religious figures in the Christian era.

The mounds piled furthest from the shore were true islands and could only be reached by boat or canoe. Unlike the crannóga near the lake’s edge, which were something of a compromise, the deep-water platforms clearly put status and security first. This is often reflected in their size, architecture and use. Evidence for small jetties and ports can be found on a number of these monuments, usually facing the nearest dry land (as with the causeways and bridges). In some cases while the rest of the platform may have been left open or with minimal protection the port or “harbour” would have featured an imposing entranceway of fencing and gates. In general, many mid-lake crannóga would have featured substantial revetments along the base, topped by fencing, protecting not just against intruders but also the elements. These were also the ones most likely to require regular maintenance and were at greater risk of abandonment.

The role of the crannóga

The purpose of each individual crannóg varied greatly in time and place. Some undoubtedly functioned as off-shore pens for small livestock, probably pigs, goats and geese, the location providing some safety from predators, be they human or animal. In such cases a simple wattle fence or timber palisade, a metre or more high, around the edge of the raised platform would have kept the animals inside (though some places might have forgone even that much). Interior fencing would have created open pens while some shelter could have been provided through small barns or lean-to huts. However, this was likely temporary rather than permanent, intended for overnight stays or during periods of inclement weather, disease or passing danger from raiders or wolves. Longer use for animals would have been problematic, predicated on the size of the island, ease of access and other local factors. Conditions on a crannóg, despite attempts at drainage and the creation of a floor of sand and pebbles, possibly strewn with reeds or straw, were invariably damp, and animal waste would have added considerably to the poor footing underneath. Maintaining the well-being of confined livestock would have required regular trips by causeway or boat for feeding and watering, or to remove excess effluent (probably pushed or dumped over the side of the artificial island). Of course, this routine would have been made easier if the herders were housed with their charges.

Close confinement between different sexes or species might also have caused problems. During mating season some animals needed separate pens for males and females unless confined there for breeding purposes, while domestic pigs were almost as hostile as their wild boar cousins. Cattle would have been especially difficult to bring to and from a crannóg unless a weight-bearing causeway was present or the island was in shallow, fordable waters (in which case some sort of long slipway would have been required from the raised platform down to the lakebed). Though we cannot be certain, practical evidence makes it unlikely that the islands, regardless of size, housed cows or bulls for any great period of time, at least before the war-ridden 15th century.

While it may look the part, there is very little evidence that the Irish or Scots constructed their crannóga on stilts. Instead they were bedded down onto the floor of the lake, which certainly provided far greater stability in the harsh environment of north-western Europe
While it may look the part, there is very little evidence that the Irish or Scots constructed their crannóga on stilts. Instead they sat on the floor of the lake, which provided far greater stability in the harsh environment of north-western Europe where storms, rains and heavy flooding were commonplace

Other crannóga undoubtedly functioned as mainly human habitats, for extended families of farmers or their aristocratic overlords. A few of the larger lake dwellings supported substantial free-standing structures, from timber walls to well-made houses, indicating long-term use. However this does not guarantee that these locations were occupied at all times throughout the year, or even from year to year. Some may have been associated with seasonal accommodation, suitable during the summer months when the crannóga may have been less prone to damp (while the winter would have seen them left to the cold and the gales). Or they may have served as one of several widely spaced palaces used by important regional kings during frequent progressions around their subject territories.

Whatever the case, the buildings on the islets for most of their history conformed to the same styles used in non-maritime settlements, invariably the well-known Irish or Celtic roundhouse and its derivatives. However the structures in Ireland were somewhat smaller than their European counterparts. These were laid out using the rope method described above, creating a circular floorplan on which a woven wall of rods was crafted. This was gradually built up to form the shape of a tall round-bottomed bowl, with the wall and roof almost seamless. When erected this would have looked something like an upside-down basket sitting on the compacted earth. The only opening would have been a narrow, cramped doorway, sometimes with a low bottom lip or jam retained from the weaving process, designed so that one had to slightly step in and out over it (this prevented the ingress of draughts, rain or even small animals). This entrance invariably faced the east or south-east, partly to avoid the prevailing westerly and south-westerly winds, but also to provide heat and light from the morning sun inside the dark structure for the early rising inhabitants. For the Medieval Irish and Scots (and their ancestors before them) the day began and ended with sunrise and sunset. However there may well have been some ritual significance to this location too, fragments of which survive in the early indigenous literature. Across the entrance was placed a door of plank or more likely wickerwork, possibly quite thick for both security and weatherproofing reasons, though the poor might have foregone this.

Following the initial stage of construction an external wall was placed around the first, stopping at the top of the inner wickerwork just before it curved up to form the doomed roof. The space between the two lines of wicker was packed with insulating material, wool and fur, while some effort was made to ensure that the rods used in the weaving were placed in such a manner that the sharp or cut ends always faced the hidden cavity, leaving smoothly woven walls inside and outside the house. Buildings in early Ireland do no not seem to have used the wattle and daub technique prevalent elsewhere in Europe, contrary to many modern claims (this is a process where the wickerwork walls are covered on the inside and outside with a mixture of insulating clay, straw and animal dung, a practice which might well have offended early Medieval Irish sensibilities).

A thatched roof was then added to the dome using interwoven and layered reeds, rushes or even heather, which would have given the top of the house a distinctive bulged appearance and a high degree of waterproofing and heat retention (some buildings may have used wooden shingles, probably cut bark, though the weight would have been a problem). Irish stories indicate that brightly-coloured feathers may have been added to some roofs, for insulation or as decoration, perhaps as a sign of wealth. Though the claims may be fanciful they remind us that these structures may have been far more ornate than we can assume form archaeological finds.

A teach or Irish roundhouse, typically sized, waiting for its thatched roof. One of the few modern reconstructions to get it right (Image: UCC)
A teach or Irish roundhouse, typically sized, waiting for its thatched roof. One of the few modern reconstructions to get it right (Image: UCD)

By creating an inner and outer shell for the building the lifetime of its organic core was greatly extended. The roof, external wall and outer filling materials, open to the elements, could have been fixed or replaced with relative ease, leaving the interior intact. The absence of load-supporting posts, beyond those in the double-wall, also ensured that there was a more open and usable living space inside. On the other hand the lack of internal joists did limit the size of what could be erected. The wider the circumference of the walls, the higher the resultant weaved dome of the roof. After a certain point, if the diameter was too great, the roof would have been too high to be reached for construction, or even to support itself (especially with the weight of a rain-soaked thatch). The maximum space the Irish builders seem to have been comfortable with was ten metres in diameter, though the vast majority of dwellings fell in the five to six metre range.

Contrary to modern suppositions, roundhouses did not employ open or louvred smoke-holes. These would have been quite dangerous, creating air-driven vortexes inside the structure that would have sucked up sparks and even flames. Instead the smoke from any internal fires was allowed to percolate up through the roof, in the process drying it out, sterilising the materials and deterring vermin or pests. During bad weather a well-insulated roundhouse probably steamed like a pot of boiling water. However one disadvantage of this method was the problem of trapped smoke lingering at or just above head height inside the building. To limit this, the fire pit was always located in the very centre of the house, where the doomed roof was at its highest so that the smoke could collect at the tallest point. The larger houses of the rich probably had the added benefit of less smoke pollution due to their higher roofs (and possibly larger entrances) but respiratory problems must have been an issue across all classes, at least for infants and children.

Every inhabited building had a fire, usually a square or oblong pit, sometimes stone-lined. Around this was placed benches or other forms of seating while sleeping cubicles and storage areas were located along the walls in a sort of petal shape. Some houses may have had movable screens for privacy or to define certain areas, though these might have been for the better off. If extra room was required a second roundhouse could be attached to the rear of the first one, creating a figure of eight pattern. This extension, usually slightly smaller, was known as the cúlteach “backhouse” or the cuile “backplace”, and was reached from the main house through a small doorless entrance with a lower lintel facing the main doorway (a hole was probably cut in the rear double-wall of the original house, big enough to step through while crouched over but small enough to still maintain the structural integrity of the wicker-work). By this method primary dwellings could be enlarged without the need for costly rebuilds or larger houses. As for the extensions, they seem to have served as kitchens, storerooms and even private bedrooms.

What we cannot know is how durable organic buildings were in the moist conditions of the artificial islands, where condensation would have been present to some degree, even in the best insulated of places. Would the central hearth have been sufficient to keep things dry? Would floor coverings and other materials have required regular replacement? It was only much later in the Middle Ages, around 800 CE, that oblong houses began to creep in to the country influenced by the mainstream European architecture of the Scandinavian-Irish towns and the Christian monasteries. These brought with them an increasing preference for buildings using stone, wattle-and-daub, lime-washing, timber planking and so on. In the space of two centuries the traditional roundhouse would almost disappear from use in the crannóga and their dryland equivalents.

In some cases the sustained use of a crannóg can be deduced not from its structural remains but from the waste that the inhabitants created during their own lives. This is especially true of so-called middens, prehistoric rubbish tips, which lake-dwellers inadvertently created by simply dumping their refuse into the water on one side of their island homes. These mounds yield a wealth of analytical materials, and simply put, the greater the amount of refuse the greater the length of human occupation. In a similar vein, dedicated cesspits, Medieval toilets, are also found on crannóga, some dug down from the edge of the raised platform into the watery subsoil beneath. A purpose-made space for human waste implies a prolonged need and some cesspits show clear signs of maintenance or refurbishment. Both of these features raise questions about health and sanitation in the damp and possibly mouldy conditions of the lake-dwellings.

One positive aspect, though, may have been the relative lack of rodents. Rats are not native to Ireland. The highly destructive black species were brought to the country by the Scandinavians or Norman-British during the Medieval period though they did not flourish in any great numbers (they were eventually all but out-bred by the disease-spreading brown rat, imported in the 18th century). Mice on the other hand are indigenous and though they can swim they tend to avoid water, especially if it is deep or turbulent. Unless they were carried there by accident, in animal feed or building materials, most crannóga would have been free of larger vermin. This was a definite advantage over land living.

There is no doubt then that only the wealthy or powerful were likely to have invested in the maintenance of large and well-equipped islands, a time-consuming effort that could only be justified by some perceived advantage gained over life on dry land. In the case of those crannóga displaying simpler habitations it can be assumed that they belonged to the ordinary people or were for occasional use and so required nothing of any prestige value. Not unrelated to this was the possible use of some lake dwellings as secure centres of industry, mainly technological in nature, with evidence of metal-working and other production processes. The structural remains on these platforms may reflect workshops and storage areas, implying a narrow band of use excluding habitation.

Related to the above, a small number of crannóga, especially those predating Christianity, may have been constructed for ritual or ceremonial functions or may have been adapted for those uses. These include assemblies associated with royal coronations, communal festivals or even treaty negotiations between neighbouring tuatha. It’s likely that the popularity and continued use of certain lake dwellings may have been linked to local folk-memories and traditions largely lost to us. By the Medieval period a number of saints – and their immediate followers –  were living in seclusion on several natural or artificial islands. Meanwhile a few wealthy monasteries were undoubtedly using the lake dwellings in much the same manner as secular communities: as permanent or temporary abodes, places of refuge or storage, pens for livestock, gathering and meeting points, shelters for hunters and fishermen, or places of worship and ceremony.

The legacy of the crannóg in the Irish landscape

The last hurrah of the crannóga took place from the 15th to 17th centuries as the islands and their offshoots provided the native Irish with moated residences, fortresses, treasuries, depots for weapons, prisons and hospitals. The more isolated, the greater the advantage, as the country groaned under the weight of English invaders and colonists. In some cases, artificial islands abandoned centuries beforehand were restored and expanded, the age of musket and cannonball requiring the addition of defensive walls of turf and stone in some places. The last recorded evidence of the crannóga in active use comes from the 18th and 19th centuries, when poor vagrants or armed opponents of British rule hid on their bleak, usually overgrown surfaces. Though some 1,500 artificial islands have been identified by historians and archaeologists, mainly in the midlands, north-west, west and north of the country, many more remain to be discovered. Very few indeed have been subject to a thorough or modern investigation by professional researchers. As much as we know about the long history of the crannóg in Ireland, and Scotland, there is much more to learn.

A surviving crannóg monument on a lake in Ireland, one of some 1,500 known from around the country
A surviving crannóg monument on a lake in Ireland, one of some 1,500 known from around the country. Many of these are now under threat form modern framing techniques, land reclamation or construction work. Several sites are damaged or destroyed each year


    1. Unteruhldingen is somewhere I want to go. I was in Stuttgart for a few days last year (work-related) and I had planned to travel there for the weekend before flying out but the company wouldn’t rejig the flight home 😦

      The Alpine lake-dwellings are somewhat different to the Irish and Scottish examples. Most obviously they were built on stilts rather than artificial islands. The timber platforms stood on wooden piles well above the surface below. Secondly they were built over bogs or marshes rather than open water, relying on the height to lessen the effects of damp and flooding. Finally they were far more liable to deterioration or loss. The crannóga may have been wet, mouldy things at times (though we cannot be sure if some form of effective damp-proofing was used, such as lime-plaster, reed-beds, etc.) but they were far more endurable than their German or Swiss cousins.

      Both are fascinating features. The reconstructed stilt-houses below represent a rather over-the-top interpretation from the early 20th century, owing more to Scandinavian inspiration than archaeology. The location is wrong, in water rather than a marsh, the height is questionable, and the architecture is (highly) debatable. I’m still aching to visit, though.

      1. I was amazed to see that there were apparently quite many, substantially different, cultures building these in what is today Slowenia, Italy, Austria, Switzerland, and Germany. There are actually some (near the Traunstein / Traunsee in Austria, and the Neuenburger See in Switzerland, most notably) that were on mounds, very similar to the crannóga, some of the available literature actually making the comparison [Die Nähe des Ufers ihre Structur und die ihnen allen gemeinsame geringe Tiefe liessen eher darauf schliessen
        dass es künstliche Inseln waren die man im See gründete ähnlich den irischen Crannogen was auch erklären würde warum sie überall gleich tief sind – “Die Pfahlbauten des Neuenburger Sees”
        By Édouard Desor]. They are rarely addressed directly in any recent documentation, however, I guess they are not as “sexy” as the houses on stilts, especially those you mentioned on the Bodensee – Very impressive. Some colourful brochures can be found here http://www.palafittes.org. I love this part of your blog. GRMMA.

    1. No probs, will be expanding it in a while with more info, just to keep it all in one place. Also, will be looking at other Medieval settlement types and housing in the near future. Might do a time-line bit as well. The internet is FULL of misinformation on the subject. Not surprising when my most recent book purchase on the subject cost me an eye-watering €60… 😦

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