The Ulster Scots were folks from Scotland planted in what is now Northern Ireland by King James I after 1600. This part of Ireland was the last part to be subdued by England. The Irish chieftains boogied to the Continent and the best land was given to these Presbyterian (and Anglican as well) settlers. The native Gaelic speaking Catholics were driven to the hills. In the 1730s 1740's and beyond thousands of these Presbyterians from Ulster (that provicne in Ireland) came to Ameica because the Presbyterian church wasn't favored either by the English. These people settled what is the frontier PA (beyond Philly),N. Carolina, western Virginia, some of upstate NY, and especially what became Kentuck & Tennessee. They were fiercely independent and joined george Washington's army in droves. This is just an overview---probably a lot you can find.
As I tell my wife, when she asks how I can be such a bull-headed pain in the arse, some of my ancestors were hand-selected by the British to annoy the Irish.