Yes, these split (or combined) townlands happen when a civil parish boundary goes through a townland. The parish boundary is here. I could possibly split Castlehill, but I'm not sure if it was decided what to do with these. As far as I know, it could be split into "Castlehill (Crossmolina)" and "Castlehill (Addergoole)", reflecting the parishes they belong to. On the other hand, someone might argue that the official names ("Castlehill" twice) should be used. They could also have been merged at some point. It's complicated!