So if we go back to the medieval period, Ireland and Gaelic Scotland are a cultural unit, speaking the same language and wearing the same clothing. This does not include the kilt in either place. It was common, however, for people to wear a mantle, a large piece of cloth, usually wool, as outerwear.

Through the centuries language use drifted to form the distinct languages of Irish and Scottish Gaelic. Methods of wearing the mantle changed, too. The first reference to kilts that we have in from an Irish text in 1594, which comments on the distinctive way that Scottish Gaelic mercenaries fighting in Ireland were pleating and belting their mantles. So we know from that that Irish were not wearing the belted plaid or they wouldn’t have considered it noteworthy.

Plaid is a Scottish Gaelic word for blanket or cloak. It didn’t have to be tartan, there are artist depictions of them in plain or striped cloth. So men began pleating the lower part of the garment and holding the pleats in place with a leather belt. The upper part of the cloth could be wrapped and worn in a variety of ways, depending on the weather, what the wearer was doing and his personal style preference. This garment is known in English as the great kilt. Women continued to wear unbelted and unpleated mantles.

Eventually the pleats would be sewn in and the garment separated into two, forming the small kilt as we know it today. But that is a development of the late 18th century, possibly connected to the industrial revolution and an increase in factory work in Scotland.

The introduction of the kilt into Ireland appears to be part of pan-Celtic efforts in the early 20th century. Although people who speak each Celtic language (Irish, Scottish Gaelic, Manx, Welsh, Cornish and Breton) have different histories and customs, they all share a history of struggling to survive in the face of English or French dominion over the places where they were spoken. In the late 19th century there was a movement to bond over both this history and linguistic similarities. It seems some of these pan-Celticists seized on the kilt as a visually striking symbol of not-Englishness, and began to introduce kilts into Ireland. This is not a period I am super familiar with so hopefully someone else will provide more detail or correct me here.

So was the kilt worn in Ireland in ancient times? No, although the garment that developed into the kilt was. Has the kilt been worn in Ireland for the past hundred years? Yes.