General
Celtic Language Source – An informative site on both Celtic language branches including Irish Gaelic (Gaeilge)
Gaelic Dictionaries – Links to Manx, Scottish, and Irish dictionaries
A Beginner’s Guide to Irish Gaelic Pronunciation
Cornish
Agan Tavas – Organization promoting the Cornish language
Irish
Oideas Gael – A County Donegal based language camp, offering a wide variety of Irish medium classes.
Daltaí na Gaeilge – A North American based Irish language organization.
Tir na nÓg – An informative site on the uses of Irish along with links and resources.
Gael Linn – An Irish language organization offering music, literature and more through Irish language medium.
Scottish
Sabhal Mór Ostaig – A Further Education college in the island of Skye, Scotland, teaching through the medium of Gaelic.
An Comunn Gaidhealach-America, Inc. – A volunteer-run, non-profit organization that strives to promote and preserve the Scottish Gaelic language and culture.
Welsh
The Welsh Language Board – Established by the Welsh Language Act 1993. Its main function is to promote and facilitate the use of the Welsh language.
Cymdeithas Madog – Maintained by Cymdeithas Madog, the Welsh Studies Institute of North America, and is dedicated to helping North Americans learn, use and enjoy the Welsh language.
Geiriadur Prifysgol Cymru – University of Wales Dictionary of the Welsh Language. In Welsh and English.
Nant Gwrtheyrn National Language Centre – Offers Welsh language courses for learners of every level, whatever the needs.