Or maybe they were just incredibly antisocial and felt compelled to leave by their desire to not live near other people. Such a seclusionist mindset might explain why they were there in the first place...
It's the same thing. If we’re going to laud Iceland for being the settlement of a terra nullius, then clearly those Irish monks had claim to it, and the subsequent settlement of the Norse without their invitation represented encroachment on someone else’s land.
Maybe it was 'trespassing' I suppose. I don't know much about this subject but from what's been described in this thread, assuming there must have been violence seems like a leap.