The interesting thing is that "Remmick" isn't an Irish surname. It's of German origin, derived from "Roemmich".

It's also worth noting that Remmick doesn't start using an Irish accent nor singing Irish songs until AFTER pretending to be a poor southern blues singer has failed.

There was an article written about that very thing when the movie first came out.

https://www.thewrap.com/sinners-irish-vampire-meaning-history-explained/

I can't speak to why the Irish instead of some other group. But generally, they're an example of a historically subjugated people who experienced hardship and discrimination when arriving in the US, but were eventually (more or less) accepted and brought into the fold as "white". Meanwhile, black people, on whose backs the US was literally built, many of whose lineage in the US stretches back hundreds of years further than many white people, still face some of the worst treatment and outcomes of any minority group in the US.

So my read is that while they had a common plight, the relative leg up the Irish had would render attempts to form community between the two groups fraught. And Remmick is clearly trying to exploit that common cause for his own purposes, and isn't too concerned with actual liberation, as demonstrated by his first recruits being Klan members.

I'm guessing by demonization you're referring to the head vampire being Irish?

I'm guessing the idea was to have someone "lower class" based on how Irish immigrants were treated in that era, e.g. poorly.

When the head vampire and the couple first try to enter the building by playing music they are sent away.

If the head vampire had been black there might have been less suspicion and he would have been invited in, which would change the film.

If the head vampire had been white/non-Irish there might have been more suspicion. Why is this group coming here instead of somewhere else in the town, something the film even brings up.

They are however just the right level of "lower class" that Halie Steinfeld's makes an effort to get to know them and possibly invite them in. That moment is key to the films plot, and an Irish immigrant fits the role well.

Other immigrants or "lower class" might have worked, but then you have the musical aspect. The Irish have a rich musical history. The preacher's son is being sought out for his musical abilities. It fits well.

Also, something to consider, while turning into a vampire is often viewed as a negative, within the film they make an argument that it isn't that bad. Stack and Halie Steinfeld live a nice life as vampires, even inviting the preacher's son to join them in his old age. Sure a vampire kills you, but maybe it's not that bad.

Why do you think the film "picked on" Irish immigrants? If anything, it was actually sympathetic towards Irish Catholics because it acknowledged the discrimination they have faced. That was the whole point of the allegorical connection between vampirism and cultural assimilation.

Racism, religious discrimination, classism, take your pick of bigotry.

I haven't seen the movie, but the Irish were viewed pretty poorly throughout American history, so that might be something that was played up in the movie. Again, I haven't seen it, though, so I don't know for sure.