The Famine emigration put a great many Irishmen in America just as the war began in 1861. Most lived in the North and served the Union, many in Irish regiments under green flags. Others fought for the Confederacy. Sometimes they faced each other across the same field.
How many served: the figures differ
There is no single agreed count. The sources measure different things (Irish-born, Irish Catholic, Irish-descended), and even one article gives two Confederate figures.
For context, the Union army article says about a quarter of the white men in the Union army were foreign-born, and that the Irish were the largest foreign-born group in the Confederate army.
People
Thomas Francis Meagher · Union · 1823–1867 · born Waterford city, Waterford Young Ireland leader of 1848, transported to Van Diemen's Land, later in New York. Commissioned brigadier general to lead the Irish Brigade in 1862; resigned in May 1863 after Chancellorsville. Drowned in the Missouri River in 1867 as acting governor of Montana Territory.
Michael Corcoran · Union · 1827–1863 · born Carrowkeel, near Ballysadare, Sligo Colonel of the 69th New York Militia, who in 1860 refused to parade it for the Prince of Wales. Captured at First Bull Run; on release raised the Corcoran Legion. Died 22 December 1863. A monument to the Fighting 69th was unveiled at Ballymote in 2006.
Thomas William Sweeny · Union · 1820–1892 · born Cork, Cork Lost his right arm in the Mexican–American War and served as a Union general; in 1866 commanded the Fenian invasion of Canada.
Thomas Alfred Smyth · Union · 1832–1865 · born Ballyhooly, Cork Union brigadier general, mortally wounded in April 1865; the last Union general killed in the war.
James Shields · Union · 1806–1879 · born Altmore, Tyrone Union general whose troops at Kernstown in March 1862 inflicted Stonewall Jackson's only tactical defeat of the campaign, though Shields, wounded the day before, was not on the field.
Patrick Cleburne · Confederate · 1828–1864 · born Ovens, Cork Former British Army soldier who became a Confederate major general, called the 'Stonewall of the West'. In 1864 he proposed freeing enslaved men and enlisting them to secure Southern independence; it met polite silence, and one general attacked it as an 'abolitionist conspiracy'. Killed at Franklin, 30 November 1864.
Richard W. Dowling · Confederate · 1837–1867 · born Knockballyvishteal, Milltown, Galway Houston saloon-keeper who commanded the largely Irish Davis Guards at Second Sabine Pass in 1863, the most one-sided Confederate victory of the war.
Irish units
Irish Brigade· Union Authorised September 1861 around the 63rd, 69th and 88th New York; later joined by the 28th Massachusetts and the 116th Pennsylvania. Meagher was its first commander.
69th New York Infantry ('Fighting 69th')· Union Fought at First Bull Run under Corcoran, then became the core regiment of the Irish Brigade, carrying a green flag with a golden harp.
28th Massachusetts Infantry· Union Boston's second mainly Irish regiment; its cry was 'Faugh a Ballagh' (Clear the way).
116th Pennsylvania Infantry· Union Recruited among Philadelphia's Irish in summer 1862 and assigned to the Irish Brigade.
Davis Guards· Confederate A company composed primarily of Houston Irish; the Davis Guards Medal, presented by the ‘ladies of Houston’, is regarded as the only medal authorised for wear on the Confederate uniform.
Emerald Guard, 33rd Virginia· Confederate An Irish company of the Stonewall Brigade; the article says it may have been first to give the 'Rebel Yell', at First Bull Run.
Fredericksburg and Marye's Heights, 13 December 1862
The Irish Brigade assaulted the sunken road in front of Marye's Heights, came close to the Confederate line under heavy fire, and was driven back by staggering casualties. Its two Wikipedia articles describe the losses differently:
A story told in Michael MacDonagh's The Irish at the Front, quoted in the Wikipedia article, has an Irish regiment on each side of the Rappahannock singing the same song by their campfires the night before the battle.
The Medal of Honor
Wikipedia keeps a category of Irish-born Medal of Honor recipients ↗, many from the Civil War. It is not a sourced list sorted by war, so we have not counted it or picked names from it. That gap is ours to fill later from a proper source.
Quick answersHow many Irish fought in the American Civil War?
Estimates differ. Wikipedia's article on the Irish in the war gives about 150,000 Irish-Americans for the Union. For the Confederacy, figures of 20,000, 30,000 and about 40,000 all appear in Wikipedia articles, counting different groups (Irish Catholics, Irish or Irish-descended, Irish-born).
What happened to the Irish Brigade at Fredericksburg?
On 13 December 1862 the brigade assaulted the sunken road in front of Marye's Heights. The battle article says 545 of the 1,200 Irishmen who charged were killed or wounded; the brigade article says its fighting strength fell from over 1,600 to 1,036.
Did Irish soldiers fight for the Confederacy?
Yes. Fewer Irish lived in the South, but six Confederate generals were Irish-born, the highest-ranking being Patrick Cleburne of Ovens, Co. Cork, and Irish companies such as Houston's Davis Guards served.