The Irish used to be in a similar position like the people from South America today. Now they are seen as white but before WWI they weren't seen as white by the WASPs. And it's totally normal for some of the second or third generation immigrants to become racist against new immigrants. Rite of passage.
To your earlier point: Boston racism is now legendary (see Celtics fans)
Many of the most disgusting and radical Democracy hating people in Trumps inner circle are Catholics by the way. Go figure.