The Irish were another major group of immigrants in the United States in the years preceding the Civil War. Unlike the Germans, the Irish were almost all Roman Catholic, and were almost all poor. The Irish came across a great deal of prejudice in the United States, based on real or imagined concerns. They were the largest group of Roman Catholics to enter the country within such a short period. This Catholicism was viewed as a threat to the Protestant majority. In addition, the fact that so many had come from poverty made them appear undesirable to the Americans, who valued financial and material progress and an optimism which they did not perceive in the Irish immigrants.

Irish immigrants had been coming to the United States since before the Revolutionary War. By the time of the Civil War, the Irish American community had made significant contributions to the United States. For example, one of the signers of the Declaration of Independence, Charles Carroll of Carrollton, was Irish; and one of the Presidents of the United States, Andrew Jackson, had been the son of Irish immigrants.

In the 1840s, however, immigration from Ireland increased tremendously. Up to that point, Irish immigrants had come to the United States to escape the political and economic inequities caused by British power in Ireland, as well as to seek more economic opportunities. In the 1840s, however, Ireland was attacked by a massive potato famine, which influenced many of the Irish to immigrate to the United States simply for survival. Between 1840 and 1860, almost two million Irish people came to the United States. Most of these immigrants settled in cities like Boston and New York. Since many were poor, they could not afford to purchase farmland, so they had to find employment in the cities. In addition, the organization of the religion to which most of them belonged, Roman Catholic Christianity, required that they remain close enough to each other to be able to attend church regularly. Many of the newer Irish immigrants worked in factories, mills and mines, and in construction gangs to build canals and railroads.

During the Civil War, the number and fervor of Irish Americans fighting for the Union helped ease their acceptance into American society. The issue of national unity was what inspired the Irish American community, rather than any widespread concern over slavery. According to the US Sanitary Commission report of 1869, 144,221 Irish-born soldiers and officers served in the Union forces. This was proportionately greater than their number in the general population. The largest number of officially-recorded Irish Americans came from New York (51,206), Pennsylvania (17, 418), Illinois (12,041), Massachusetts (10,007), Ohio (8,129), Missouri (4,362) and Wisconsin (3,621). Irish units fought to declare their American patriotism, while proudly proclaimed their Irish roots and displaying slogans and items to that effect. The "Irish Brigade," a collection of New York, Massachusetts and Pennsylvania regiments, was the most famous of these "Fighting Irish."

The most prominent Irish officer was probably Michael Corcoran. His father had been an officer in the British army, and young Corcoran emigrated to the United States in 1849. In 1859, he became colonel of the 69th New York Militia; but lost the position the next year, when he refused to parade the regiment before the Prince of Wales, who was visiting. He was spared a court-martial because the beginning of the Civil War made him an important asset to the US Army because of his ability to raise Irish volunteers. Corcoran became a hero at the First Battle of Bull Run, but was captured. Upon his release, he was commissioned a brigadier general and was invited to have dinner with President Lincoln. Corcoran continued his efforts to recruit Irish Americans for the Union, and raised the Corcoran Legion, which was also called the Irish Legion.

After the Emancipation Proclamation of 1863, however, the Civil War was increasingly perceived as a war against slavery, rather than a war for the Union. This made it unpopular among many in the Irish community, which was generally antipathetic toward abolition. In New York, in the summer of 1863, many Irish Americans were hostile to African Americans, some of whom had replaced Irish longshoremen on strike. When the draft drawing of that summer listed a majority of Irish men, many were angered. Some of the Irish Americans in New York, who were also resentful of the war policies which allowed the wealthy to get out of military service, protested against the draft. The protests led to rioting, one of the most severe domestic upheavals in American history. Casualties were estimated at anywhere from 300 to 1,200. Several African Americans were lynched, and the Colored Orphans Asylum was destroyed, although all the children were evacuated from the building. Ironically, many of the police officers trying to establish order were themselves Irish Americans, as were many of the people trying to rescue the children in the orphanage an help restore law and order. Draft riots took place in other cities in the North, but none other had the same ferocity as the New York riots.

By the end of the Civil War, however, the conspicuous service of thousands of Irish troops in defense of the Union helped establish Irish Americans more securely as Americans. They had the right to vote, and were able to obtain political power largely because of their high urban concentrations. While they still faced many derogatory stereotypes, and had to endure the prejudice and anti-Catholic sentiments of other Americans, the Irish were eventually assimilated into the mainstream of American culture.