It is estimated that more than 150,000 Irishmen fought for the Union in the Civil War, and no group was more renowned than Meagher’s Irish Brigade. Convicted of treason and banished to the penal colony of Tasmania, Meagher staged a dramatic escape to the United States in 1852. Once the Civil War began, Meagher enlisted in the Union Army and began to recruit his fellow Immigrants to join him in an all-Irish unit within the 69th New York Infantry Regiment. 
The Irish Brigade led the way in many of the Army of the Potomac’s major battles, and Meagher’s men paid a heavy price. At Antietam, they charged into battle under the Gaelic cry of “Faugh-a-Ballagh” (“Clear the way!”) and shouts of “For Ireland!” and “For St. Patrick!” During the bloodiest single day in American history, the Irish Brigade lost 540 men—half of its force—as casualties. 
Antietam was the single bloodiest day, the number of Americans (North and South) that died that day were more than the Revolutionary War, Mexican War and the War of 1812 combined. Both sides lost a combined total of 22,720 casualties in a single day. Lincoln considered Antietam a victory since the South withdrew south of the Potomac River and issued the Emanicpation Proclamation, freeing the slaves in all states in active rebellion (slaves in states that remained in the US were not freed at this time) 
The attrition suffered by the Irish Brigade eventually took its toll on Meagher, who resigned his commission several months after meeting with Lincoln. Kelly, a native of Ireland’s County Galway, assumed command of the Irish Brigade and led them into the slaughter at Gettysburg in July 1863. More than 200 of the brigade’s 530 remaining soldiers were lost. Kelly would be killed in June 1864 during the siege of Petersburg before receiving the official promotion requested by Lincoln.