Yep, it seems that the kilt as a symbol of Irishness is a product of a marketing effort into "pan-Celtic" sentimentality. The Irish themselves never really adopted the idea except for a few ardent fanatics, and some military units.

But that doesn't really matter, does it? Here in the USA, where you live, wearing a kilt to represent Irish descent will be perfectly acceptable. In fact, when I'm out and about in my kilt, the most frequent question I'm asked is whether I'm Irish.