Immigration from Ireland to what is now the United States can be broken down, essentially, into two waves. The earlier wave was mainly of Protestants from Ulster. Many of these were descendants of relatively recent arrivals to Ireland (~200 years prior, mainly from lowland Scotland), and not all were fully assimilated into native Irish culture.* This is one of the reasons why Northern Ireland remains separate from the Republic of Ireland to this day. The second wave is the one more familiar to Americans, that of Catholics from across the island fleeing economic disaster and the infamous Potato Famine. I recently watched the documentary The Irish Empire, which had interesting facts about both waves. They only link together by involving people from Ireland, but I’ll give them both here.
First, the series points that out among the largely Presbyterian Ulster Irish, a.k.a. Ulster Scots, Scotch-Irish, or Scots-Irish, the long training to become a preacher of that denomination led to shortages on the American Appalachian frontier. The gap was filled by Baptist preachers who required far less training, and to this day the Appalachian region is heavily Baptist. These immigrants were never really Catholic, and the Catholic Church was not a very influential presence in the English-speaking colonies. This phenomenon is partially addressed in Albion’s Seed, but I don’t remember that it presented the facts in just this way. This would probably help to explain a good deal of religious change in other times and places.
Second, in some American cities in mid 1880s and beyond, Catholic Churches used Irish cultural sentiment to maintain the interest of their heavily Irish congregations. These particular city folk belonged to the wave that made Irish and Catholic largely synonymous. A similar phenomenon happened in Australia, where almost all Catholic education was mixed with Irish culture. Interestingly, it was at this time that the term Scotch-Irish was used by the older wave to distance themselves from these new immigrants who were almost all at the bottom of the social ladder. Before the second wave, the first wave and their descendants were simply referred to as “Irish”.
* Exactly how assimilated they were is still something of a mystery to me. Judging by music alone they must have been at least partially assimilated. My paternal grandfather’s family came from this wave, if I’m not mistaken, and they considered themselves Irish, celebrated St. Patrick’s Day, etc. It wasn’t until later in history, i.e. after the first era of mass migration to the Western Hemisphere, that the apparently impenetrable cultural divide was driven between the territories that are now Northern Ireland and the Republic of Ireland. However, the fact that there was at least some religious and social difference in the first place demonstrates that the mixing was not 100%.
P.S. I didn’t include it at first because it was not new to me, but it’s relevant to point out that most people of Irish descent in the United States are descended from the first wave and not the second. When John F. Kennedy was hailed as the first Irish-descended president, what it really meant was that he was the first from the second wave. The interpretation is that he was the first authentically Irish-descended president, because by that time the descendants of the first wave had lost or given up the cultural battle in being seen by outsiders and by themselves as “Irish” pure and simple.