The Catholic Irish have been a major American ethnic group since they started arriving in the 1830s in the wake of the horrible Irish Potato Famine. Extremely poor with a strong group cohesiveness, high interest in politics, and high rates of alcoholism and violence, they settled in the slums of large cities. many worked in construction gangs. The Irish rose rapidly in the ranks of the Democratic party, the Catholic Church, and (after 1880), the labor unions. The Irish were very well organized, and, since 1850, have produced a majority of the leaders of the Catholic Church in the U.S., labor unions, the Democratic Party in larger cities, and Catholic high schools, colleges and universities. Politically, the Irish Catholic typically voted 80-95% Democratic in elections down to 1964.
About 45 million Americans are of Irish descent--but a majority are Protestants.
Irish Catholics began arriving in the 1830s and especially 1840s, in the wake of devastating famine in Ireland. (Others went to Britain, Canada and Australia.)
Very high group cohesion enabled labor bosses to organize unskilled workers into construction gangs and move them to locations wehere canals and railroads were being built. Native Americans rarely did this work (although in the South slaves were used, and in California Chinese.) The Irish fought for the jobs against other Irish work gangs.
Most went to the largest cities, especially Boston and New York, as well as Chicago, San Francisco, Saint Louis, Hartford, Albany, Philadelphia and Detroit. In the Northeast, Protestant owners of texile mills fired the Yankee women working there and replaced them with cheaper Irish women.
Many served in the Civil War (on both sides) but they resented the draft and rioted in New York on a major scale in 1863.
Irish in politics and government
The Catholic Irish moved rapidly into law enforcement, and (through the Catholic Church) built hundreds of schools, colleges, orphanages, hospitals, and asylums. Political opposition to the Catholic Irish climaxed in 1854 in the short-lived Know Nothing Party. The Irish vehemtly opposed blacks and abolitionists before and during the Civil War, and led the battle to stop Chinese immigration in the 1870s. The most important organization in the late 19th and early 20th century was the Ancient Order of Hibernians. Irish dominated the Knights of Columbus, but it was open to Catholocs of all ethnicities.
Crime and Police
The Irish Catholics formed gangs that used violence to control neighborhoods and work places (so that only gang members could get jobs). Combined with high rates of drunkenness and disorder, he gangs meant the Irish crime rate was very high.
The most famous violent gang was the Molly Maguires who terrorized the coal mining camps of Pennsylvania until broken up by hanging 18 leaders in 1877.
By the 1850s, the Irish Catholics were a major presence in the police departments of large cities. In New York City in 1855, of the city's 1,149 policemen, 305 were natives of Ireland. The creation of a unified police force in Philadelphia opened the door for the Irish in that city. By 1860 in Chicago, 49 of the 107 on the police force were Irish. Chief O'Leary headed the police force in New Orleans and Malachi Fallon was chief of police of San Francisco.
John F. Kennedy was their greatest political hero. Al Smith, who lost to Herbert Hoover in the 1928 was the first Irish Catholic to run for president From the 1830s to the 1960s, Irish Catholics voted 80-95% Democratic, with occasional exceptions like the election of 1920, when they angrily turned away from the Democrats who had failed to secure Irish independence.
Ronald Reagan boasted of his Irishness. (The son of an Irish Catholic father, he was raised as a Protestant.)
John F. Kerry was the losing Democratic nominee in 2004. Joe Biden is the only Catholic elected vice president. In 1944 Jimmy Byrnes lost the Democratic nomination for vice president because he had been born Catholic and left the Church, making him ineligible in the minds of many Catholics.
Today, most Irish Catholic politicians are associated with the Democratic Party, although some have become Republican leaders, such as former GOP national chairman Ed Gillespie, New York Congressman Peter T. King and retired Illinois Congressman Henry Hyde.
Historically, Irish Catholics controlled many city machines and often served as chairmen of the Democratic National Committee, including Thomas Taggart, Vance McCormick, James Farley, Edward J. Flynn, Robert E. Hannegan, J. Howard McGrath, William H. Boyle, Jr., John Moran Bailey, Larry O'Brien, Christopher Dodd, and Terry McAuliffe. The majority of Irish Catholics in Congress are Democrats; currently Susan Collins of Maine is the only Irish Catholic Republican senator.
Exit polls show that in recent presidential elections Irish Catholics have split about 50-50 for Democratic and Republican candidates; large majorities voted for Ronald Reagan. The pro-life silent majority in the Democratic party includes many Irish Catholic politicians, such as senator Bob Casey, Jr., who defeated Senator Rick Santorum in a high visibility race in Pennsylvania in 2006. 
Many major cities have elected Irish Catholic mayors. Chicago, Boston, and Jersey City have breen especially prominent. New York City has had at least three Irish-born mayors and over eight Irish Catholic mayors. The most recent one was William O'Dwyer, elected in 1949.
By the 1890s the Irish largely controlled the Catholic Church in the U.S. Most bishops and college presidents were Irish. Sometimes this involved battling other ethnic groups, such as the Germans (who outnumbered the Irish), the Poles and the French Canadians.
Boston College, established by Jesuit priests in 1863 to appeal to urban Irish Catholics, offered a rather limited intellectual curriculum. The Jesuits of the late 19th century were wary of the radically changing world and limited intellectual study to Thomistic philosophy. The priests prioritized spiritual and sacramental activities over intellectual pursuits. One consequence was that Harvard University would not admit Boston College graduates to its law school. Jesuit interaction with the world of authentic scholarship would not come until well into the 20th century.
The American Irish Catholics were strong supporters of independence for Ireland, and from the 1860s to this day were the main financial backers of independence movements, followed by movements to disrupt British rule in Northern ireland. In the 1860s several thousand Civil War Union veterans formed the Feniean Society, which planned a military invasion to seize canada. The plan was to trade Canada for a free Ireland. The Fenians were too few and too badly organized; they failed miserably. The Americans played a decisive role in the Northern Ireland peace process in the 1990s, ending the Irish Republican Army's campaign of violence.
- Glazier, Michael, ed. The Encyclopedia of the Irish in America, (1999), the best place to start--the most authoritative source, with essays by over 200 experts, covering both Catholics and Protestants.
- Anbinder, Tyler. Five Points: The Nineteenth-Century New York City Neighborhood That Invented Tap Dance, Stole Elections and Became the World's Most Notorious Slum (2001).
- Bayor, Ronald and Timothy Meagher, eds. The New York Irish (1996) comprehensive overview by numerous scholars
- Blessing, Patrick J. The Irish in America: A Guide to the Literature. (1992)
- Clark, Dennis. The Irish in Philadelphia: Ten Generations of Urban Experience (1973)
- Diner, Hasia R. Erin's Daughters in America: Irish Immigrant Women in the Nineteenth Century (1983).
- Dolan, Jay P. The Irish Americans: A History (2008), survey by scholar
- Erie, Steven P. Rainbow's End: Irish-Americans and the Dilemmas of Urban Machine Politics, 1840—1985 (1988).
- Gleeson; David T. The Irish in the South, 1815-1877 University of North Carolina Press, 2001
- Greeley, Andrew M. The Irish Americans: The Rise to Money and Power. (1993).
- Ignatiev, Noel. How the Irish Became White (1996). New Left
- Jensen, Richard. "No Irish Need Apply": A Myth of Victimization," Journal of Social History 36.2 (2002) 405-429
- Kenny, Kevin. The American Irish: A History (2000), solid scholarly history
- Lee, J. J., and Marion Casey, eds. Making the Irish American: History and Heritage of the Irish in the United States (2007) excerpt and text search
- McCaffrey, Lawrence J. The Irish Diaspora in America (1976).
- Meagher, Timothy J. Inventing Irish America: Generation, Class, and Ethnic Identity in a New England City, 1880-1928 (2000).
- Meagher, Timothy J. The Columbia Guide to Irish American History. (2005).
- Miller, Kerby M. Emigrants and Exiles (1985), influential study
- Mitchell, Brian C. The Paddy Camps: The Irish of Lowell, 1821—61 (1988).
- Mulrooney, Margaret M. ed. Fleeing the Famine: North America and Irish Refugees, 1845-1851 (2003). Essays by scholars
- Negra, Diane, ed. The Irish in Us (2006). Focus on use of Irish symbols in popular culture
- O'Donnell, L. A. Irish Voice and Organized Labor in America: A Biographical Study (1997)
- Potter, George W. . To the Golden Door: The Story of the Irish in Ireland and America (1960). useful survey online edition
- Potter p.530
- He had only one Irish Catholic grandparent but most people considered him Irish.
- [George J. Marlin, The American Catholic Voter (2004), pp 296-345
- Prendergast, William B. The Catholic Voter in American Politics: The Passing of the Democratic Monolith (1999)
- The Poles grew so angry that a minority faction in 1897 split off from the Roman Catholic Church and set up the Polish National Catholic Church in America, which today has about 150,000 members.
- Marianne Gallagher, "The Jesuits at Boston College in the Late Nineteenth Century," American Catholic Studies 2007 118(2): 43-66,
- By contrast, when Ulster Protestants tried to mobilize support in the U.S. in the 1970s, they had minimal success. Andrew J. Wilson, "Ulster Unionists in America, 1972-1985," New Hibernia Review 2007 11(1): 50-73,