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% 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, and 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.