People who think of themselves as "Celtic" musicians also play music originating in areas like England, Central France and Shetland, where there's little Celtic heritage, and North America, where there is a Celtic heritage but it's diluted with other cultures. The term is approaching meaninglessness -- it now means something like "music that people who like Scottish and Irish traditional music also like". It's becoming no more than a convenient label for record stores to use -- but the same thing happened to the term "folk music", didn't it?