The plaid/tartan wearing is much broader than Scotland, and naturally you could identify an individual's home to some degree by the tones they achieved by usign their native plant and vegetable dyes. Hence the clan tartan concept, and the kilted Pan-Celtic identity.

Kilts supposedly developed from a number of influences, but crystallized in the highlands. I consider them native Celtic dress, because they developed this way long after Ireland had its native dress banned by England, whereas before the banning Scotish and Irish dress were remarkably similar. It wouldn't be a stretch to therefore consider kilts a natural evelution of Gaelic fashion, hence the Irish desire to participate in kilt wearing.

Since then kilts have become identified with a broader "Pan-Celtic" identity, as opposed to the Germanic identity of Britain's past. People are wanting to say England is more Briton than Anglo Saxon, which would imply Celtic roots. Studies on this are interesting, and the kilt is ever the ready to be used as an icon of Celtic resistance against Germanic invasion, opression, etc. This is more critical than it sounds -- one of the arguments for annexing Scotland was that its native people, Picts, were Germanic, thereby making Scots look like barbarian invaders and encouraging the sons of Germanic tribes to take action. Picts being Germanic is likely a falsehood, but you see how the lines are drawn and symbols become attributed to one group or another.

Also, I love the Romans detailing the cothes of the Pretani people. ^_^