I do understand your point on the example of the Donegal and Kerry examples, Nerd, as they are both geographically and musically at opposite ends of the spectrum, as Kerry fiddle bowing tends to be more legato than the staccato, aggressive Donegal bowing, and that polkas and slides are not types of tunes native to Donegal, and germans, highlands, and lancers are not tune types native to any other part of Ireland except for a few Northwest counties. They are very distinct styles with little in common, but I still hear both styles as being very "Irish." The Donegal/Scottish connection seems a little like the Scottish/Appalachian connection, how Scottish tunes became "Irishized" in the hands of Donegal fiddlers (taking strathspeys, for example, and changing the rhythm), and how they became "Americanized" when they crossed the ocean. Another example that tells my ear that two separate styles are both very much part of a national music, is the West highlad/Eastern styles in Scottish music. The highland styles, strongly influenced by pipe music and "rougher" sounding, and the more refined, classical-influenced Eastern styles, are, like the Donegal/Kerry example, very disparate, but still, they both have those qualities that make both styles very much Scottish; very much a part of their national music.
   One more example that comes to mind is Shetland fiddling. Even with Irish influence, not to mention it's norwegian and Scottish roots, and, in the 20th century, swing jazz influence in accompaniment, the music makes one prick up the ears and say "that sounds like Shetland music."
So, this is a long way of saying that, taking into account all the different influences that shaped these countries music, whether over many years or in ecent times, and some very disparate styles within these countries, I think the "national" character of this music; what makes it "sound Scottish," etc. still holds true.

Chanteyranger