Which religion is this

So I’m trying to figure out what religion my neighbours could be. We are in Ireland and they are an Irish family. A priest would come to their house every night and they would pray late in to the night ( their son told my daughter this) the priest only wears all black clothes, not typical priest gear but just black pants and black shirt. Over the last few weeks he seems to have moved in with them, his car is always there and you only see him leave occasionally. It’s wrecking my head trying to figure out what religion they could be. Definitely not Catholic. Any ideas??