Final answer:
When Irish Catholic immigrants started to arrive in the US in the 1820s, they mainly settled in East Coast cities as laborers, faced significant discrimination, and contributed to the increase in the Roman Catholic population amid nativist hostility.
Step-by-step explanation:
Irish Catholic immigrants began arriving in the US in the 1820s, and several significant events and conditions characterized their experience. First, they settled primarily in the East Coast cities, finding employment as laborers in a burgeoning urban environment. Intense discrimination was a common struggle for these immigrants, both socially and economically.
Secondly, the arrival of Irish Catholics contributed to a sizable growth in the Roman Catholic population of the United States, which was relatively small before this period. Thirdly, their settlement in the US coincided with nativist sentiments, leading to a sense of competition and hostility from other working-class citizens who were concerned about job security and cultural changes.