That's not being crabby Mick. I think the one thing the marketing term "celtic music" gives us it at least some areas of music to look at. To quote a little from the rec.music.celtic FAQ:

"Generally understood, "Celtic music" refers to the folk musics of Ireland, Scotland, Wales, Brittany, (Spanish) Galicia, Cornwall, the Isle of Man, and certain parts of Canada (esp.Newfoundland/Cape Breton), as well as more recent musical styles (i.e. rock, jazz, new age) that draw heavily from these folk traditions. It should be understood, though, that not all musicians who hail from Ireland/Scotland/ Wales/Brittany/etc. necessarily play Celtic music."