Thomas William Sweeny was born in Co Cork on 25 December 1820. He immigrated to the United States in 1833. In 1846, he enlisted as a second lieutenant in the 2nd New York Volunteers and fought under General Winfield Scott in Mexico. Sweeny was wounded in the groin at the Battle of Cerro Gordo, and his right arm was so badly injured at the Battle of Churubusco that it had to be amputated. His fellow servicemen nicknamed him “Fighting Tom”. Despite his wounds, he continued serving with the 2nd U.S. Infantry until the outbreak of the Civil War.
At the outbreak of the war, Sweeny was in command of the arsenal at St. Louis, Missouri. In reply to Confederate sympathisers who tried to make him surrender the arsenal, he declared he would blow it up rather than surrender. He participated in the capture of Camp Jackson in May 1861 and assisted in organising the Home Guards.
Sweeny commanded the 52nd Illinois at Fort Donelson. At Shiloh, in command of a brigade, he successfully defended a gap in the Union line. He was wounded in the battle having received two shots in his only remaining arm and a shot in one of his legs. He kept the field until the close of the fight, exciting the admiration of the whole army.
He returned to command of the 52nd Illinois but returned to brigade command when Brig. Gen. Pleasant A. Hackleman was killed at Corinth. He commanded the Second Division of the XVI Corps during the Atlanta Campaign. At the Battle of Atlanta, Sweeny’s division intercepted John B. Hood’s flank attack. Sweeny got into a fistfight with his corps commander, Maj. Gen. Grenville M. Dodge, when Dodge broke protocol and personally directed one of Sweeny’s brigades during the fight. Sweeny received a court-martial for these actions but was acquitted. He mustered out of the volunteers in August 1865, and was dismissed for going AWOL by the end of the year.
In 1866, he commanded the ill-fated Fenian invasion of Canada, after which he was arrested for breaking neutrality laws between the United States and Britain, but was soon released. He was reinstated with his former rank of major later that year, and retired from the Regular Army in May 1870 as a brigadier general. Sweeny retired to Astoria on Long Island. He died there on 10 April 1892.