The persecution of the Irish was a leftover of the British's attempted cultural eradication of celtic and Catholic identities. British media often portrayed the Irish as violent and backwards, and the living conditions the British enforced often made the stereotypes seem true. After the Eiri Amach of 1798 and Irish Potato Famine of 1845 there were two waves of large immigration of Irish farmers, fisherman, and sailors to Boston and New York. These former farmers, fisherman, and sailors were extremely unfamiliar with many of the job types available in the heavily industrialized cities and struggled to find employment they couldn't form for themselves. As a result high levels of unemployment and street crime were part of American Irish life in cities in the 1850s and 1860s, leading to them being seen as troublemakers and being heavily targeted by city patrolmen.

By the 1870s a large portion had left through Pennsylvania to settle in the Appalachian Mountains, and many more were pushing further west to work on the rail lines. As they were often paired up with the Chinese and German migrant workers they were distrusted and weren't easily integrated into heavily English, French, and Italian descended communities that settled along the developing railroads, continuing the reputation of the Irish being supposed troublemakers.

However back in the major east coast cities the Irish who stayed were successfully carving out their own districts thanks to the enforced isolation from other ethnic groups, allowing them to form almost vertical control of the political process from individual home to district level. To ensure this control wasn't ceded as the cities grew and to prevent the return of the abuse of the 1850s, rising political institutions like Tammany Hall heavily encouraged first and second generation Irish immigrants to perform enforcement instead, leading to Irish descendants taking positions as everything from police officers to prosecutors. By the 1910s this push meant that as many as one in five police officers in New York and Boston was either an Irishman, the child of an Irishman, or the grandchild of an Irishman.

There are a couple of different things coming together but the big one is that while they were first considered degenerates, criminal, drunks, etc. they weren’t denied all of the legal status whiteness offered. As states removed the property requirements, allowing all white men to vote, the large groups of Irish immigrants voted cohesively enough to become very influential in a lot of cities – helpfully around the same time that booming cities needed professional police & fire departments, creating a ton of civil servant jobs which did not require formal certifications or uncommon skills. Once a few people from a tightly-knit community get in more will follow, and the Irish tended to be more insular as Catholics in a Protestant country.