The whole US Police Department was created to deal with the so called "Irish problem"

Northern cities use to relied on a volunteer watch system, in which male citizens served a few hours a week. That wasn’t sufficient to handle changing urban dynamics with approximately 1 million Irish arriving from Ireland. Beginning in the 1840s, Ireland’s rotting potato crops drove hundreds of thousands of its people to flee to the United States. The Irish used to make up a majority of criminals in America mired in extreme poverty as a result of centuries of British rule. You had Anti-Catholic, anti-Irish mobs destroying houses and torching churches in the deadly Broad Street Riot because formal police squads didn’t yet exist.. Boston created the nation’s first full-time police department, followed by New York in 1845. But there was barely a pretense of professional training or discipline among these early police forces. There was a lot of just random beating up of people to ‘keep order,’.. The new police departments shifted the role of law enforcement from apprehending criminals to preventing crime by proactively controlling the “dangerous classes,". In many cities, Irish-Americans were considered a prime example of a “dangerous class,” which meant no one was going to hire them to work let alone in the nascent police departments. In 1890 Irish had IQs 20% lower than American-born Protestants.

Mass hiring of Irish police officers would had to wait until Irish voters gained political power in the cities, which they did over the next few decades. Given the large and growing Irish populations in many urban areas, Democratic Party leaders quickly found that it was a good idea to seek their votes. “How do you get votes?” Dwyer-Ryan says. “You do favors. You get them jobs.”

Eventually, the hiring of a few Irish policemen led to many more, as cops helped their friends get jobs. Barrett notes that the structure of Irish-American life lent itself particularly well to this kind of networking. Close-knit Catholic parishes and county organizations—based on where Irish members’ families came from—functioned as employment networks.

But even as the Irish came to play an outsize role at city halls and police departments, they were also major forces in street gangs and organized crime all the way into the early 20th century. Even now there are some really extreme cases where you even find [gangsters and cops] in the same family.

At the same time, more Southern and Eastern European immigrants, and African-Americans from southern states, were arriving in northern cities, creating new tensions for Irish cops. They still beat up Irish people—Irish suspects—but they’re also dealing in a more hostile way with newcomers. During riots, it was common for Irish police to join forces with Irish mobs against less politically powerful Italians, Jews or African-Americans.

Over time, these groups—and others that came after them—did their own political organizing to gain power in city government and police departments. Meanwhile, Americans with Irish heritage spread far beyond the cities and now number more than seven times higher than the population of Ireland itself (32 million Americans have predominantly Irish roots). But Irish-Americans, who began their rise to power at the very birth of modern policing, still maintain a huge presence in many police departments to this day.

Sources

https://www.history.com/news/when-america-despised-the-irish-the-19th-ce...

https://www.history.com/news/how-stereotypes-of-the-irish-evolved-from-c...

https://www.historyplace.com/worldhistory/famine/america.htm