Americans have long been fascinated by the Civil War, marveling at the size of the battles, the leadership of the generals, and the courage of the soldiers. The Civil War was the deadliest conflict in American history, and had the two sides realized it would take four years and inflict over a million casualties, it might not have been fought. Since it did, however, historians and history buffs alike have been studying and analyzing the military and political history of the conflict ever since.

Given the extent of Irish immigration in the 19th century, it should come as no surprise that the Irish played important roles in the Civil War. While exact numbers are not known, the most commonly cited figures are 150,000 Irish serving in the Union Army, 20,000 or so in the Union Navy, and 20,000 more in the Confederate military. Immigrants were 13 percent of the total American population in 1860, and in the 1840s alone, 780,000 immigrants came from Ireland, almost half the total immigration for that decade. In the 1850s, 914,000 immigrants came from Ireland, a third of the total. The great majority of them settled in the North’s big cities, particularly New York City, Boston, and Philadelphia, while much smaller numbers settled in Southern cities like Richmond, Charleston, and New Orleans. 

The Irish were notoriously discriminated against when they arrived. Nativist sentiment against immigrants, particularly the Irish, grew and peaked intensely by the mid-19th century, with the anti-Irish and anti-Catholic Know Nothing Party achieving considerable political success in the 1850s. One reason the Irish were so strongly Democratic is that the Democrats more readily accepted immigrants, and the remains of the Know Nothings migrated into the newly founded Republican Party. Irish votes gave the Democrats control of many Northern cities. 

In the process, the Irish experienced a great deal of violence at the hands of nativists, including churches and homes looted and burned in riots. They formed self-defense units to defend themselves, which easily converted to companies in regiments being recruited when the war began. The first two Union soldiers killed in the war were both Irish, as was the last general on either side, shot by a sniper two days before Lee’s surrender. Several of the Irish units achieved fame, particularly the Irish Brigade, composed of several New York Irish regiments. 

The Irish joined for many reasons, including wanting regular wages and enlistment bonus money, defending the Union, and even hoping that veteran Irish soldiers could help free Ireland from British rule. Some of the veterans after the war went on to participate in the 1866 Fenian invasions of Canada.

There were 18 Irish-born generals in the Civil War, with 12 in the Union and six in the Confederacy, and some of them were quite colorful characters. One Union Irish general was one-armed and was court martialed for getting into a fistfight with another Union general. Another Irish-born Union general had challenged Abraham Lincoln to a duel when both were young politicians in Illinois. This same general was elected to the Senate from two different states before the war, and he would be elected to the Senate from a third state after the war. 

The Irish in the North at first strongly supported the war, but as the war went on and casualty lists grew longer, enthusiasm faded. Most of the Irish immigrants in the big cities were fearful that freed slaves would come north and take their jobs, so they were opposed to Lincoln’s Emancipation Proclamation. They were also generally opposed to the draft because it allowed a draftee to obtain a deferment by a $300 payment to the government, which was called “commutation”. Draftees could also hire another man to take their place, which was called hiring a “substitute".