Matt wrote
In the twentieth century, you have the kilt being adopted by Cornish nationalists, Welsh nationalists, etc., as well as many Irish, as a way of identifying themselves as "celtic" (read, non-English), primarily (in my opinion) because the kilt was such an easily recongizable "celtic" form of clothing.
I think that this establishes it. But, marketing ploys aside, I think it's moving on from here even. There's a rising level of desire to find a general Celtic identity - which may well be separate from the nationalisms - and the kilt is becoming its outward manifestation.

This seems to be taking place across the Celtic home nations of the British Isles (Scotland, Wales, RoI and NI, Cornwall, and IoM) and even gaining ground in Brittany and Galicia. What the reasons for this may be would need a political scientist to examine - probably it's got something about not wishing to disappear into amorphous pan-national political arrangements.

Anyway, if they want to adopt the kilt that's fine by me, as long as it is recognised as being an import from the Scottish highlands and islands - a gesture to some sort of pan-celtic identity - and not try and make of it something that it's not.