Refugees fleeing famine, chaos, and an oppressive political regime en masse. They went to countries where locals objected to their presence. They were considered undesirable aliens who would never assimilate because they were illiterate spoke a different language, and practiced a strange religion...
they traveled on unsafe ships, which they called "coffin ships" because of the high mortality.
if this sounds familiar, maybe it's because it is your own history.
Irish History Podcast has a series on this.