Modern, I bought it on the internet. Origin 1980s punk movement.

1800s Kilts legalized and are all the rage in Victorian England. "Clan Tartans" invented.

1700s Kilts outlawed and forgotten. Bummer dude.

1700, The little kilt invented in Scotland by cutting the great kilt in half.

1500s the great kilt invented in Scotland by buckling a belt on the outside of a brat.

Prior to this in Ireland the nobles wore leinte and brats IE: linen tunics and wool wraps with edgeing in solids, stripes and MAYBE tartans. Belts worn over the leine but under the brat. Meanwhile the Scots are said to dress a lot like the Irish. Beyond this we are guessing.

Conclusion: While the kilt has ancient roots in Celtic brats, the kilt as we know it is Scottish only. So if the Irish want to wear ancient Celtic garb then they need to find a yellow nightshirt and wrap a striped blanket over it. As for the Welsh, ancient wood cuts clearly depict Tom Jones wearing tight pants and a loose shirt.