CELT: Corpus of Electronic Texts: a project of University College, Cork, College Road, Cork, Ireland http://www.ucc.ie/celt. This is a major project to make texts available online. Links to some of the particular sections are included below.
CELT (Corpus of Electronic Texts) Index of Captured Texts. This is the general index to texts which have been "captured" (i.e., scanned electronically). Includes links to lists of Hiberno-Irish texts, French Texts, Irish Texts, Latin Texts, and Translated Texts.
CELT (Corpus of Electronic Texts) Translated Texts. There are scores of primary sources translated into English and available online here in html and plain text formats.
CELT (Corpus of Electronic Texts) published documents. This page links to previously published texts that have been made available online. Some are in Latin, Irish, and other languages than English. Some have been translated into English. Including the Annals of Ulster, The Annals of the Four Masters, The Second Battle of Moytura, and many others.
CELT Bibliography of Bibliographies compiled by Donnchadh Ó Corráin (November 2003). A listing of 123 bibliographies and library catalogs in Celtic Studies.
Celtic Literature Collective. Maintained by Celticist Mary Jones of Philadelphia. This is an attempt to collect as many possible early and medieval texts produced in the "Celtic" countries, or on Celtic themes (hence the inclusion of Continental Arthurian works).
Internet Medieval Sourcebook. Maintained by the Fordham university medieval studies department. Primary texts in medieval areas mostly other than Celtic (e.g., the full text of Chaucer's Canturbury Tales); excerpts for use in teaching European medieval history, philosophy, and literature.
MS-OMIT (Manuscript Sources to Old and Middle Irish Tales). A list of Early Medieval (until ca. 1100) narrative texts in Irish language with (a) standardized references to their medieval and early modern (not modern) manuscript sources and (b) information about their availability in print.
Primary Sources: Irish Literature
The Cattle-Raid of Cooley (Táin Bó Cúalnge). This HTML edition pairs an adaptation of the English translation of Joseph Dunn (1914) with the Irish transcription of Ernst Windisch (1905.)
Primary Sources: Welsh Literature
The Mabinogion. Translated by Lady Charlotte Guest. Online text from Project Gutenberg transcribed from the 1849 edition.
Taliesin Pages. Introduction to the poet Taliesin and complete translated texts of the Hanes Taliesin, the Llyfr Taliesin, Black Book of Carmarthen, and the Red Book of Hergest.
Primary Sources: Literature Generally
Project Gutenberg. The premier online collection of electronic texts of works in the public domain. Texts are offered in plain text format. Click here for the Gutenberg Search Page to search their database.