The Irish were the first Catholic group to immigrate to America en mass, and they suffered vicious discrimination because of it. But, while many settled the land, even more settled in cities, and over time, they gained control of municipal politics and controlled services like police, fire, sanitation, etc., when appointments of cops, firemen, etc., were made through political patronage. Irish also controlled the Catholic diocese of most cities and appointed pastors and administrators. So, when Italians immigrated to American cities later, they found themselves constantly having to go to the Irish for the basics--and they resented it.

By the time Italians swarmed to America, Irish were already well established (though by no means free of discrimination), so the Italians were on the bottom--and suffered even more discrimination. Most American bigots (and there were plenty) refused to consider them "white." That added to the friction beweeen Irish and Italians.

In my old neighborhood in Brooklyn, we had a riddle:

Q. How does the newspaper article reporting on an Italian/Irish wedding begin?

A. "Among the injured were..."