My mother says that tensions between Irish and Italian Americans were actually quite tense in 1970s Philadelphia. She insists that an Irish person marrying an Italian person was actually somewhat scandalous in some more ethnic neighborhoods, and that Irish Americans generally got along much better with WASPs and Jewish Americans.
However, Irish and Italian Americans are both obviously Catholic. Irish immigrants arrived several decades earlier, though. Did this 20th century animosity have a 19th century basis, or did Irish immigrants welcome more Catholics to America?
How did Catholic Germans, Hungarians, and Poles fit into all of this?
What a brilliant question. I too would love to know more about this; I'm of Irish-Italian descent myself, except the norm in my part of the world was that the two communities had considerable solidarity, with Catholicism playing quite a big role in that. This would be a fascinating contrast if it's true.
As a follow-up (though perhaps too specific for anyone to really know): what would the actual practice of Catholicism have been like where the two communities were close? Would there have been much in the way of shared worship, and if so, would this have been a source of healing or further tension if the communities had mutual animosity?