I do hope Kevin won't mind me using his words; but, I think McGrath of Harlow answered this question in this thread back in May:

There's Gaelic music, which hails from Scotland and Ireland and the Isle of Man and has a fair number of things in common musically; then there are other types of Scottish Music and Irish Music, which are not Gaelic.

And there's music from Britanny; and music from Wales, where the language is pretty similar, but the cultures and the music have very little in common. And there's music from Cornwall, where the old language was similar again, but the culture has very little,in common with either Britanny or Wales. And down in Spain there is Galicia where they speak Spanish, but the people may have spoken some kind of Gaelic related language.

The point I'm making "Celtic" music as such, in the sense of a common heritage and tradition stretching over all theese countries, doesn't really exist. What there is is a bunch of very different traditions with some great music, and it's always good to exchange ideas and tunes and such.