The kilt as we known it today is most certainly Scottish, as was its ancestor, the Feileadh Mhor/Belted Plaid. Before that, we're told Highlanders dressed in "the Irish fashion." So, in a roundabout way, the kilt has made its long descent from Irish clothing to Highland clothing to, now, pan-Celtic clothing.

When selecting a "national costume", Irish nationalists chose to create things like the solid green kilt or the saffron kilt. While partially based on historians' misconceptions of the léine and other Irish wear, it is interesting that people like Pádraig Pearse chose something (perhaps without realising it) that was ultimately based on their own historical clothing. The modern Irish saffron kilt thus represents the léine (through its colour) and the old belted plaid. It is nice to see Irish culture borrowing from Highland culture, since of course, it was usually the other way around.

You can be sure tartan was worn in the other Celtic nations as well. It would not be a stretch to say the ancient Welsh wore tartan mantles/wraps/capes.

So, we have the modern, tailored kilt as a Scottish national garment dating from the late 18th to early 19th Century, as a Irish national mode of dress starting in the mid- to late 19th Century, and as a Welsh/pan-Celtic garment starting in the mid- to late 20th Century.

All Celts should be proud that the Scottish Highlanders managed to sustain the only form of traditional Celtic dress that has survived into modern times.

P.S. That reminds me, experts (and most official organisations) generally agree that there are only six Celtic nations: Ireland, Scotland, The Isle of Man, Wales, Cornwall, and Brittany. Claims put forward by regions like Galicia, and Asturias (both in Spain) have little validity, as their claims of "Celtic" culture do not meet any definable criteria.