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.