Irish Catholics emigrating to the U.S. were singled out for exclusion due to a mix of religious, ethnic, and economic prejudice. Many Americans at the time were Protestant and viewed Catholicism with suspicion, fearing loyalty to the Pope over American institutions. Additionally, Irish immigrants were often poor and competed for low-wage jobs, leading to resentment from native-born workers. Their large numbers and distinct cultural identity further fueled nativist fears that they would not assimilate into American society.