dil.ie - Electronic Dictionary of the Irish Language - "The electronic Dictionary of the Irish Language (eDIL) is a digital dictionary of medieval Irish. It is based on the Royal Irish Academy’s Dictionary of the Irish Language based mainly on Old and Middle Irish materials (1913-1976) which covers the period c.700-c.1700 but incorporates corrections and additions to thousands of entries."
www.celtic-languages.org/Old_Irish/Resources - Old Irish resources, books, etc.
www.celtic-languages.org/Old_Irish/Guide_to_Old_Irish_spelling - Guide to Old Irish spelling.
www.omniglot.com/language/numbers/oldirish.htm - Numbers in Old Irish.
omniglot.com/language/numbers/celtic3.htm - Numbers 1-10 in Primitive Irish, Old Irish, and Classical Gaelic.
www.celtic-languages.org/Classical_Gaelic/Resources - Classical Gaelic resources, books, etc.
www.celtic-languages.org/Notes_on_Classical_Gaelic_Grammar - Notes on Classical Gaelic grammar.
léamh.org - Early Modern Irish texts, glossary, grammar, and paleography.
A Student's Companion to Old Irish Grammar, by de Vries (2020)
Old Irish Grammar and Reader, by McCone (2005)
A Concise Old Irish Grammar and Reader, by Pokorny (1914)
Old-Irish Workbook, by Quin (1975)
Sengoídelc: Old Irish for Beginners, by Stifter (2006)
Ogam: Language, Writing, Epigraphy, by Stifter (2020)
Early Irish, in The Celtic Languages, 2nd edition, by Stifter (2009)