My players have been asking this question for a while now. Ireland in 1641 doesn't appear to have any real reasons to have such an influx of demons, neutral spirits, and other supernatural creatures. Now, I actually have an explanation for this.
Yes, you heard that right, Angels. More specifically, Metos, but it ties into all of them.
The campaign world exists in a multiverse where a predatory corruption bounces from universe to universe, attempting to transform each of them into some version of hell. Hell, for campaign purposes, is a universe that has fallen to demonic corruption and now threatens Earth. The two universes (Hell and the one the players are trying to save) are intersected somehow, allowing travel between them.
Travel from Hell to Earth and back is difficult. The majority of demons are incapable of traveling to Earth without risking a permanent demise, although especially wise/old/strong specimens have learned to project avatars of themselves to Earth to reduce the risk of their excursions. Losing an Avatar might be painful, but it doesn't cause any permanent damage at least.
Angels are also from another universe, one that managed to stave off the corruption and achieve a horribly high level of Ultra-tech. Using this technology to transform themselves into terrifyingly powerful androids- and later, pure machines- they took the fight to Hell.
Needless to say, the various religions of the world have no clue about this.
The Angels are limited in number. The vast energy required to create just one could kill stars and blacken solar systems of light. Only five are active on Earth. Metos, the strongest, is fully capable of destroying entire planets, but that only serves to temporarily stem the tides of Hell.
The Angels don't have the means to create more of themselves to the same degree of power that they already possess. Hell is always expanding, always creating more troops, many of which are truly expendable.
It's a real Zerg Vs. Protoss with humans caught in the middle situation. (Disclaimer: I'm a massive Starcraft fan).
Metos is the most powerful Angel, the one capable of crushing planets and killing stars. It's his presence in Ireland, in a state of rest over a particularly powerful ley line that is the origin of the numerous incursions into Hell. His presence has weakened the barrier between the universes, allowing more free travel between the two.
This isn't completely unique to Metos either- most of the angels are strong enough that they similarly start to strain the underpinnings of reality that make it harder for things to cross between dimensions.