A lot of people seem unaware of the two best online resources for historical texts relating to Ireland, both of which are entirely free to use. The first is “CELT, the Corpus of Electronic Texts”, a collection of hundreds of manuscripts and books in digitised form mainly written in Irish and English (of various periods) but also featuring works in Latin, Norman-French, German and several other languages. The 1300+ entries cover nearly one-and-a-half thousand years of literary and scholarly output on this island nation and are incredibly important, representing some 15 million words in total. The project is maintained and regularly updated with new materials by University College Cork (UCC) so you can be confident of its academic credentials. If you prefer the printed word to the electronic kind some of the texts are available through the Irish Texts Society and the School of Celtic Studies which is part of the Dublin Institute for Advanced Studies. Fair warning, many of the published texts are quite expensive (though DIAS has a sale on at the moment with a few good discounts on offer).
CELT is continuously in need of funding so if you have a few euros, pounds or dollars to spare you can donate them here.
A second and a closely related site is Irish Script On Screen, a collection of digital images of Irish and Scottish manuscripts in various languages found in the collections of several universities and institutions in Ireland, Scotland and Australia. It is stored and maintained under the auspices of the School of Celtic Studies at DIAS and is growing every year with scanned images that span the centuries from the early Medieval period to the Industrial Age. I have to admit that I love this site and I’ve spent literally hours searching through it. It will make you ache that traditional Irish lettering is no longer in popular use, either in printed or written form. Like some Arabic texts there are manuscripts here, even relatively late ones, that are almost works of art so beautiful are they to the human eye (trying to link to specific images or pages is almost impossible due to the way the site is set up, so apologies if I can’t provide any ready examples. Take my word for it and explore for yourself).
I should also mention a useful addendum to both of the above which is eDIL: the Electronic Dictionary of the Irish Language, maintained by the Royal Irish Academy and Queen’s University Belfast. It is a digitised and much expanded version of the early 20th century “Dictionary of the Irish Language based mainly on Old and Middle Irish materials” originally published by the RIA in several parts. The latest revised online edition, again free to use, is fully searchable and is genuinely groundbreaking in terms of research into the earliest literary forms of the Irish language. In a similar vein is In Dúil Bélrai, a less comprehensive but again searchable English-Old Irish glossary from Sabhal Mòr Ostaig in Scotland. There is also a very useful list of other dictionaries and resources in general kept up-to-date by the excellent SMO. For comparisons or follow-ups on particular words you can use the Foclóir, the modern Irish-English/English-Irish digital dictionary maintained on behalf of the Government of Ireland along with Focal – Bunachar Náisiúnta Téarmaíochta don Ghaeilge, a more technical database of Irish terms (the former should eventually supersede the latter). Finally there is the now antiquated but still highly useful Foclóir Uí Dhuinnín from the University of Limerick which contains lots of old words and phrases no longer encountered in vernacular Irish (unfortunately).
Hope you might find one or two of those sites interesting over the weekend.