Simply put, the kilt as we now know it evolved in the Scottish highlands, period. Later, it was embraced by Scottish lowlanders as well, as the national garment of that country.
In more recent years it has been adopted as a "pan-celtic" garment, though historically, of course, it isn't. Some Irish nationalists in the late 19th and early 20th centuries were kilt wearers, it being a symbol of anti-English "celticness", I suppose.
Others know the history of Irish kilt use as it relates to military units and pipe bands better than I do....