Over 400,000 Irish soldiers, sailors and marines fought to preserve the Union. Using the standard typically applied by the New York Times, for example, when reporting on racial tensions in South Boston or on Boston crime families, anyone with an Irish ancestor is Irish regardless of where they were born.
The number of people born in Ireland who served in the Union army and navy was about 200,000 and may have been as high as 250,000.
The number of native-born Irish serving the Union is often misreported by careless historians as 144,221. The source for this is the Sanitary Commission report, "Military and Anthropological Statistics of American Soldiers" (SCR), estimate of the nativities of the soldiers who served in the states' volunteer regiments. SCR did not include the soldiers serving in Regular army regiments, nor the Navy's sailors and marines. SCR also did not count 92,000 volunteers from California and the territories, and the militia mobilized during war's early years culminating in the battle at Bull and the Pennsylvania invasion crisis of 1863 (The Great Militia Mobilization of June, 1863).
Moreover, as the SCR report notes, the Union army did not record place of birth in its records for 43 percent of the soldiers. Not surprisingly, the highest levels of failure to record were in states with the highest concentration of immigrants: Massachusetts, New York, and Pennsylvania.
Adjusting for all the SCR's omissions brings the Irish-born total to over 200,000.
A more rigorously scientific approach is to use the Medal of Honor sample. The 1500 Medal Honor recipients provide a good sampling of the Union's soldiers, sailors and marines. This statistic is very highly correlated with enlistments and combat mortalities for a place of origin. The statistic indicates 10 percent of the enlisted soldiers were Irish-born, 5 percent of the army officers, and 15 percent of the sailors, giving a total of 210,000.
The biases in the Medal of Honor sample suggest that it might understate the Irish-born total somewhat. 1) There is award "clustering" in some of the predominantly native-born units that when eliminated raises the Irish percentage somewhat. 2) Possibly due to prejudices at the time some Irish-born soldiers may have claimed that they were American-born. A special case of this is the "Anchor Baby" effect. Specifically, individuals like Sheridan and Dennis Hart Mahan claimed to be American-born. However, they were baptized relatively shortly after their parents arrived in America. No one knows exactly where they were born.
Colonel Patrick Henry O'Rorke led the West Point class in which George Armstrong Custer graduated last. The Irish-born O'Rorke also led the 140th New York in the charge that saved the Union position at Little Round Top during the Battle of Gettysburg, although American historians have chosen to credit a Puritan college professor and a Maine regiment with the feat. O'Rorke was killed during the charge he led and did not receive a Medal of Honor.
Individuals born in America with Irish families include generals Grant, Sherman, Meade, Reynolds, and Sheridan (if he was born in America and not Ireland).
The Generals and a Small Catholic Church in Ohio
Sherman is an interesting case because he was the foster child of a devout Irish Catholic family and grew up to marry the daughter in a Catholic wedding. His in-laws played an important role in the early years of Notre Dame University.
The North's Greatest Hispanic General
Meade was the child of Irish refugees who fled the British Isles' sectarian Civil Wars in the 17th century. His grandfather became a prominent Philadelphia merchant who bankrolled Washington's starving army at Valley Forge. The British may have run the Meades out of Ireland, but the Meade's had the last laugh, helping Washington run the British out of America.
Many of the Union army soldiers credited to England, Scotland and Canada by the SCR, were in fact Irish. One of the most notable soldiers in this category was Edward Doherty who was born in Canada. Doherty fought at Bull Run and was captured while the 71st New York Militia covered Burnside's retreat. He made a daring escape from captivity, joined New York City's other Irish Brigade, Corcoran's Legion, and was appointed Lieutenant in the 16th New York Cavalry in 1863. Doherty led the 16th New York when it hunted down and killed John Wilkes Booth after the Lincoln assassination.
In the Navy
Among the many Irish who joined the Navy in the great harbor cities of New York, Philadelphia and Boston: Alfred Thayer Mahan and Michael Healy.
Alfred Thayer Mahan was the son of West Point faculty leader Dennis Hart Mahan. He graduated from Annapolis in 1859 and joined the Navy in 1861. He was at Savannah and greeted Sherman at the sea in 1864 to which Sherman replied: "So you're old Dennis's boy." Mahan's first published work reflected his Civil War service: The Gulf and Inland Waters. Mahan was president of the Naval War College in the 1880s and 90s and completed his great and widely influential works on Sea Power then. Theodore Roosevelt was a lecturer at the War College for Mahan and that relationship culminated in the birth of the modern, oceangoing U.S. Navy, Europe's lifeline to freedom in the Atlantic and America's shield against fascism in the Pacific during World War II
Michael Healy is renown as the captain of the Revenue Cutter Bear, legendary defender of native Alaskans and inspiration for Michener's Alaska and London's The Sea Wolf. Much less well-known is the fact that Healy was one of the first officers of African descent commissioned to defend the Union by Abraham Lincoln.
Gettysburg and Field Fortifications
Pennsylvania's 69th Irish Regimen and James Mallon's 42nd New York "Tammany Hall" Irish Regiment played a crucial role in repulsing the Pickett's Charge on the third day at Gettysburg.