I think I might have some answers to Matt Newsome’s questions, but I also have a question of my own. Matt wrote:-

Keep in mind, as well, that until the latter nineteenth century, no one thought of themselves as "celtic." One was a Scot, and Irishman, a Welshman, etc. There was
no sense of belonging to some overarchign "celtic culture" at the time.
Matt,

When linguists first proposed the hypothesis of an Indo-European language family and filled in the structure of the family tree, it was soon realised that Welsh, Cornish, Manx, Scots Gaelic and Irish Gaelic were all members of the Celtic branch of the tree. This was the catalyst for the term ‘Celtic’ to be applied, not only to the languages themselves, but also to the inhabitants of these lands, as if they were all members of one big family or race.

So why then do we today have the Irish, Welsh, Cornish, and Manx wearing the kilt, and speaking of it as a "celtic" garment? It was never worn by any other celtic group. I think the origin must lie somewhere in the twentieth century.

So why the change? When did the kilt become a "pan-celtic" garment? Again, I'm not saying here that non-Scots should not wear the kilt. But when did this shift occur?
I have read in a Cornish web site that some time early in the second half of the last century, the Celtic Congress agreed that kilts and tartans should be adopted by all Celtic nations to demonstrate their Celtic family solidarity. If this is correct, then the decision seems to have been taken despite that fact that there had never been any tradition of tartans or kilt wearing among the Welsh, Manx or Cornish peoples. This led to a veritable explosion of new designs of Welsh, Manx and Cornish tartans, and the Irish got in on the act by augmenting their solid coloured kilts with new tartan designs.

This may explain all of the new tartans, but one question remains unanswered. How did the solid coloured Irish kilt come into being well before the middle of the last century? I recall seeing an old black and white photograph, taken in the 1920s, of a class of teenage Irish boys in a school hall performing physical exercises, and they were all wearing dark solid coloured kilts. Does anyone know how this tradition of solid coloured kilts got started?

Rob