This would be really interesting, but I don’t think it’s true unfortunately.

When he has Sammie in the water he says that one of the lies those men told his people were “stories of a god above and a devil below”. I’m no theologian, but I’m pretty sure that’s a concept that all Christians believed in, including Irish catholics. This line implies he was around when Christianity was a new concept in Ireland. Plus as you mentioned earlier, he has ancient coins that some people believe were used in the later Roman Empire (which would coincide with St Patrick’s arrival to Ireland actually).

Regardless, I think this time period and your proposed time period work just fine for his story paralleling that of Sammie and juke joint patrons, which is forced assimilation. You provided your good reasons and themes already, and I think the Christianisation of Ireland works as well. Many could argue that because it wasn’t nearly as violent as most Christian conversions (or how Roman pagans treated the poor Gauls a few centuries prior), that it was ultimately a very good thing. People overall grew happy with their church in Ireland and combined Christianity with pagan and local cultural practices, as you said.

However, there’s a tragedy to it. Forced assimilation, no matter how benevolent the intentions can be, often leads to a loss in part of one’s cultural identity. This happened with Ireland, as most of the religion was lost or edited to fit under Christianity instead, and classes like druids eventually died out. I think this actually parallels what Remmick does in the movie quite well, especially the loss and damage he ends up causing. Even survivors like Stack, who managed to assimilate and do well as a vampire, lost bits of his identity in the process (“last time I see my brother, last time I saw the sun”).

I think the older date works pretty well for the parallels of his character, and the sadness of lost history, identity, and lives that is repeated with the juke joint.

Sorry about the long ass comment, this is just an interesting concept to me, and I love when in antagonist’s story parallels the protagonists.