Marcus Tanner recently wrote a very good modern survey of the Celtic languages called "The Last of the Celts." In it, he surveys the history and survival (and survivability) of all the existing Celtic languages, including Manx, Cornish, and Breton. One revelation the book had for me is that their fellow (Lowland) Scotsmen used to refer to Highlanders as "The Irish." Whether or not that was done in a spirit of sarcasm or was actually linguistically and culturally accurate is not for me to judge.