Appalachia

That’s going to give you a big selection bias. A lot of Appalachians are “Scotch-Irish” (also called “Scots-Irish”) who came from Northern Ireland, which is more heavily Protestant than the rest of Ireland.

Before that, their ancestors were from Scotland and northern England before immigrating to Ireland. In the UK and Ireland I believe people with similar ancestry are usually called Ulster Scots (for the Ulster region of Ireland, if that wasn’t obvious)

And in addition to that, there were probably a lot of Catholic people/families who converted along the way after arriving here since the US is overall mostly protestant of one flavor or another, and they just sort of assimilated into that or wanted to avoid anti-catholic discrimination (which has been a thing at different times and places around the US, the KKK for example has historically been very anti-catholic, and even as recently as JFK there was a decent amount of people concerned that since he was a Catholic that he’d be taking orders from the Pope or something)

In other parts of the country you’ll probably find more Catholics of Irish ancestry. Anecdotally, growing up in the Philly suburbs, myself being partially of Irish Catholic descent, I only remember one protestant Irish family being in school with me, but plenty of Irish Catholics (there may have been others, but I only remember them, we didn’t exactly go around discussing religion all that often)

In Irish history, the people there kicked out the Jesuits, the Earth Beast of Revelation 13. Why was this? The Irish Catholics knew the Jesuits’ dirty deeds.

The Irish who identify as Protestant likely understand that Popery is not correct, and had actually figured out how evil Catholicism is. Catholicism is the Mark of the Beast after all.