It depends on your definition of kilt. The skirt like article we generally wear is primarily Scottish (some interesting history there but go with Scottish).
A wrap, like the full plaid, would be common to nearly all cultures and at some point any Celtic culture would have worn that.
So, take the same evolution and any Celtic group can wear it, and then anybody can.

I wear a Batman jacket because I like it and Batman, not because I claim to be Batman.