New York’s longest-serving police commissioner, Raymond Kelly, is an Irish-American. So is the department’s current commissioner, James O’Neill. Municipal police departments across the country celebrate the role of Irish-American cops with Emerald Societies—and there’s historic reason for all of this. Through the 20th century, Irish-Americans dominated many urban police departments. To some extent, they still do today.

The flood of Irish into law enforcement in the second half of the 19th century was particularly striking because, just a couple of decades earlier, city authorities had viewed Irish immigrants as the source of a serious crime problem. In fact, to a large extent, northern U.S. cities invented their police departments as a way to control the Irish “problem.”

In the mid-19th century—and particularly after the Great Famine that ravaged Ireland in the late 1840s—families fled to America with no money to buy land, ending up in the growing shantytowns and slums of cities like New York, Philadelphia and Boston. They took the jobs they could get—as unskilled laborers or domestic servants, making very little money. Like other struggling groups before them, some turned to petty theft or sex work to make ends meet.

But it wasn’t just crime that worried the authorities. Historian James Barrett, author of The Irish Way, says anti-Catholic prejudice, combined with cultural differences, made the influx of Irish families seem particularly threatening. Irish immigrants of the era mainly came from the countryside, where a rougher way of life, including drinking and clashes between rival clans, was common. In the tightly packed urban neighborhoods of a country gripped by temperance fever, it created a powder keg.

“Most historians would agree that there was very strong prejudice” against the Irish, Barrett explains. “That translates into a lot of different things, like problems getting jobs.”

One early, violent clash came in 1837 in Boston, when an Irish funeral procession blocked a volunteer firefighting company—made up of American-born Protestants—returning from a fire. As history blogger Patrick Browne writes, the riot that followed involved 15,000 people, about a fifth of the city’s population. “Yankees” ransacked and virtually destroyed the city’s Broad Street Irish neighborhood, though the only people convicted in the wake of the riot were Irish-Americans.

Police did nothing to stop the Broad Street Riot because formal police squads didn’t yet exist. According to Marilynn S. Johnson, a history professor at Boston College, by the 1830s, southern communities had already created the forerunners of modern police departments in the form of slave patrols. But northern cities still 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.

“New immigrant groups were moving into the cities, labor conflict was developing, and gang activity was developing, pitting groups from different ethnicities and neighborhoods against each other,” says Johnson. “So you start seeing the formation of urban police departments.”

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The year after the Broad Street Riot, 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,’” Johnson says.

The new police departments shifted the role of law enforcement from apprehending criminals to preventing crime by proactively controlling the “dangerous classes,” writes Garry Potter, professor at Eastern Kentucky University’s School of Justice Studies. That often meant arresting poor city dwellers for disorderly conduct or public drunkenness.

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 in the nascent police departments.

“By the 1840s there’s all this anti-Catholic, anti-Irish fervor in urban areas, so it made sense to a lot of nativists that ‘of course we’re not going to hire Irish cops,’” says Meaghan Dwyer-Ryan, a historian at the University of South Carolina Aiken.

Historians generally agree that the country’s first Irish policeman was Barney McGinniskin, a Boston laborer hired to the police department in 1851. But McGinniskin’s career was a short, troubled one. According to Peter F. Stevens, author of Hidden History of the Boston Irish, one alderman objected to his appointment on the grounds that hiring an Irish cop would create a “dangerous precedent,” since “Irishmen commit most of the city’s crime and would receive special consideration from one of their own wearing the blue.”

McGinniskin lost his job after just three years, when the fiercely nationalist, anti-Catholic Know-Nothing Party took control of the Massachusetts legislature.

Mass hiring of Irish police officers would have 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.

Many decades after the major wave of Irish migration, remnants of that system remained in place, says Barrett, whose father was a police officer. “My father says that it was common within police departments for people to ask what county you were from,” he said. “People identified that way well into the 20th century.”

Eventually, many party machines were not just supported by the Irish but led by them. When Tammany Hall’s infamous Boss Tweed was thrown out in 1872, his successor was “Honest John” Kelly, an Irish Catholic who created a more systematic spoils system to distribute work to party supporters in New York.

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. “There are some really extreme cases where you even find [gangsters and cops] in the same family,” says Barrett.

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,” Johnson says. During riots, she notes, 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. But Irish-Americans, who began their rise to power at the very birth of modern policing, still maintain an important presence in many police departments to this day.