Well, here we go again.

Firstly, let me point out that this is X Marks the SCOT.

Next, Gaelic culture in the Scottish Highlands came from Ireland, not vicea versa. Yes, the people there are also of Pictish and Viking origin, but the language is not (to the extent it is still used), nor much of the culture. Yes, the kilt originates in Scotland, but it is derived from the Irish brat (rhymes with cart, not with cat), which was a cloak.

To confuse matters, the English sent Scots as settlers over to Ireland to deliberately change the demographics of the population, but that was hundreds of years later, and few of them were highlanders, although due to internal migration, many were of highland descent. If that weren't confusing enough, there was even later population movement from Ireland to Scotland, mainly to cities (Sean Connery and Billy Connolly are famous Scots of recent Irish descent).

There were indeed Irish clans. Clan based ownership of land comes from the Brehon law, another thing which the Gaels brought to the Highlands from Ireland. Sorry, the Scots DID NOT invent clans. Some may split hairs by insisting on calling Irish clans septs instead of clans, but AFAIK this is only based on one reference in one book, and arguing about what to call them is sophistry anyway. Cromwell forced many Irish clan chiefs off their land, and forcibly relocated some to Connaught, the Western province of Ireland, moving the chief of the Callaghans in particular from County Cork to County Clare.

As for tartan and clan tartans, that's two questions. Celts wore tartan before they ever set foot in the British Isles, and the brats (cloaks) worn by the Irish who founded the Gaelic culture in the Highlands were said to be 'striped', so probably tartan.

Most Scottish clan tartans, OTOH, were only designed hundreds of years after the clan system died out. As for Irish tartans with actual names, let alone representing Irish clans, these are even more recent. However, several registers of tartans now exist, and anyone can essentiallly register anything they like to represent any group, as long as they have some woven, so under such a system Callaghan Modern is as valid as McDoofus of Mullthwacket Muted, or WHY, and sometimes the Scottish ones are not much older than the Irish ones. I actually have cousins in Scotland whose clan tartan goes back to all of 1986 (to be fair, they are lowland or border Scots).

Irish kilts were conceived in the latter part of the 19th century, at the tail end of the celtic revival period, and were originally solid saffron or solid green, to deliberately be different from tartan kilts worn in Scotland. There is some irony in this, given that firstly the Scots got tartan from the Irish, and secondly solid colour kilts were once relatively common in Scotland. I consider the events of over 100 years ago to be history, but evidently some here choose not to.

IME it is not popular on a site with SCOT in the name to point out that tartan and clans (but not clan tartans nor kilts) and Gaelic came to Scotland from Ireland, and yet they did. Nor is it popular to say that the first kilts originated by wearing a belt over the top of an Irish cloak, and yet they did.

As for whether anyone in Ireland wears a kilt in the present day, probably only for weddings and to play the pipes. So, about as much as in Scotland, really.