www.omniglot.com/celtiadur - Exploring connections between Celtic languages, modern and extinct.
www.celtic-languages.org/Main_Page - Celtic languages resources, courses, dictionaries, media, etc.
celticstudies.wales - Books and publications.
www.evertype.com - Books in Cornish, Breton, Welsh, Irish, Scottish Gaelic, Manx, Scots, English dialects, and many more languages. Also various resources and info for Celtic languages.
www.celtic-languages.org/Category:Text_corpora - A list of useful publicly-available corpora for Celtic languages.
transceltic.com - Resources on the six Celtic Nations.
forum.unilang.org/viewforum.php?f=126&sid=885e380f8b6cbab5bc90c695d7ffc96c - Celtic language forums.