I think the proportion of people in Northern Ireland who are Ulster Scots is not as much as a half. About a third would be closer, so probably about 600,000 out of 1.8 million. Protestants slightly outnumber Catholics there, but not every protestant is Ulster Scots. The Methodists may be of English or Welsh descent, and the Church of Ireland people may be of English or just simply Irish descent. According to the stats I have read, though, there is an almost (although not exact) 1-1 correspondence between being Presbyterian and being Ulster Scots.

So how come there are more Scotch-Irish in America than Ulster Scots in Northern Ireland? Easy, really. This is a bit like the famous statistic that there are more Poles in Chicago than in Warsaw. Many (most?) Americans are a mixture of various ethnic groups. Perhaps the same person is unlikely to be both ethnic Polish and ethnic Scotch-Irish, as they settled in different places, but I have cousins in the US mid-West who are a mix of English, Welsh and Latvian, for example. IOW, each person may count many times over for a variety of ethnic groups.

People from West Virginia have told me that they regard Methodist and Presbyterian churches as interchangeable (!), but they had no idea that the Presbyterian church is Scottish in origin or that the Methodist church was brought there by Welsh settlers. It's probable that their family trees have both Welsh and Scotch-Irish in them, along with who knows what else. Many of the miners in Appalachia are of Welsh descent, their ancestors having gone from coal mines in Wales to the same work in the new world. I remember a TV documentary showing an old disused mine in Appalachia with signs prohibiting the miners from speaking Welsh, which they pointed out as racist.

As to kilts in Ireland, they aren't worn much, other than by pipers and to some weddings, but they aren't a purely or even predominantly protestant or
Scotch-Irish preserve. Kilts were worn as a symbol of Irish nationalism, including by Patrick Pierce, who took part in the Easter Rising of 1916. Treading on dangerous ground, but I couldn't let that suggestion go unchecked.