Irish men wearing kilts is a recent phenomenon.
If you research Irish history, you will find that many Scots as far back as the 16th century cast off their kilts when they came to Ireland. There are woodcuts depicting Scottish Redshanks in Northern Ireland dressed after the Irish fashion, that is, leine, Ionar, and bare legs.
The Irish “kilt” was called a ‘Mantle’ … simply a cloak of wool with a fur lining. (not plaid worn about the waist) Kilts were too cumbersome in Irish bogland and thus were often cast aside by Galloglas once in service of Irish Gaelic Lords. (Good Source Material … Elizabeth’s Irish Wars by Cyrill Falls, Tyrone’s Rebellion by Hiram Morgan, Irish Battles by G.A. Hayes-McCoy, The Irish Wars by Ian Heath and David Sque, and The Twilight Lords by Richard Berleth) Oftentimes they were made into Truibhas (Trews) or the modernday “trousers’. (Dress in Ireland by Mairéad Dunlevy) The mantle was the mainstay of Irish dress and was used like a blanket at night and functionally cast side when the whether was warm. During the end of Nine Years War between Hugh O’Neill 2nd Earl of Tyrone and Queen Elizabeth I, the English were able to recognize Irish and Old Irish “Galloglas” from the “New Scots” by the fact that the “New Scots” arriving from Scotland, were still kilted. This distinguished them from their counterparts who had been assimilated into Irish society. It wasn’t until recently that Irish people actually began to adopt the wearing of kilts.
Various claims have been made that a number of Irish tartans were shown in a book called Clans Originaux (1880) but when the list was discovered it was more of a pattern book than a literary one and not one Irish pattern was contained. Nor have any other 19th century books on tartan. Pictures purportedly showing the Irish wearing tartan kilts have turned out to be mail quilts (protection) or the bottom of the Irish leine. The only actual Irish historical tartan dated to the 17th century was found in a peat bog in the 1950s in Ulster, the tartan cloth is thought to have been manufactured in Ireland but the actual trews they were made into in Scotland. Given the movement over the sea between the Gaelic cultures of Highlands of Scotland and Ireland historically hardly proof positive that it was Irish tartan.
I lived in Ireland all my life and the first I learned about Irish and kilts was on American TV at a traditional Irish wedding. I have only ever seen Scottish people on stag nights wearing kilts in Ireland. I think some Irish-Americans may have simply gotten confused.
The Irish tartans that exist today have nearly all been invented by the Scottish tartan manufacturers in the twentieth century to sell to tourists. They are mostly taken from the colours of the arms of the counties and regions and the colours substituted into Scottish tartans to produce them. The tourist boys produce a database of Irish surnames and link it to the regions which are linked to these spurious tartans, you put your name in and as very few names have a tartan they just pop up one of the invented counties tartan, utter nonsense.
The Irish pipe bands today (usually playing a scottish bagpipe) wear a plain saffron kilt, the color just about proveable the garment not.