Sweeny, Thomas William (1820–92), US army brigadier-general and Fenian, was born 25 December 1820 at Dunmanway, Co. Cork, the youngest of four sons of William and Honora Sweeny. After his father's death in 1827, Thomas, his mother and one of his brothers emigrated to New York in 1832, where Thomas completed his schooling. He was apprenticed to a leading printer and also joined several of the city's militia companies. In 1846 he became second lieutenant of the New York volunteers in the Mexican War and lost his right arm from a bullet wound sustained at Contreras, Mexico, 20 August 1847. Returning to New York in 1848, he received a rapturous welcome and was honoured by the city. He was then commissioned second lieutenant in the US army and in the 1850s engaged in operations against the Yuma Native Americans of the southwest and the Sioux of Nebraska. In January 1861 Sweeny was promoted to captain and sent to defend the St. Louis arsenal from Confederate attack. He was appointed brigadier-general of the Missouri Volunteers (20 May 1861), and in September 1861 colonel of the 52nd Illinois Volunteers, playing a significant role in the battle of Shiloh (6 April 1862) at which he was again wounded. He was commissioned brigadier-general of the US Volunteers (29 November 1862), and in April 1865 was one of the guard of honour in charge of Abraham Lincoln's body as it lay in state in New York city.
After mustering out of the service 24 August 1865, he joined the Fenian Brotherhood and attended its national convention in Philadelphia, where he was elected secretary for war and commander-in-chief of the Fenian army in America. The constitutional revisions at the conference led to a split: one wing under the constitutionally-weakened president John O'Mahony (qv) refused to countenance any action other than revolution in Ireland, while the other led by William Roberts (qv), president of the Fenian senate, and Sweeny, advocated an invasion of Canada. Sweeny unveiled his plan for invasion at a conference of the Roberts wing in Pittsburgh, on 19 February 1866: a three-pronged invasion from Vermont, and from New York state and Illinois into Canada West (Ontario). The invading army would secure railways, canals and the seats of government and industry in London, Toronto, Ottawa and Montreal.
Fearing eclipse by the Roberts faction, O'Mahony attempted a quick military raid against Campobello, a British-controlled island between Maine and New Brunswick in April 1866, but quickly withdrew in the face of the Canadian militia, British warships, and American opposition. His failure was a boon for the Roberts-Sweeny camp which immediately moved for a larger attempted invasion. On 31 May a group of 800 Fenians under John O'Neill (qv) calling themselves the Irish Republican Army (the first time the terminology was used) crossed the Niagara River at Buffalo, New York, into Canada West, occupying Fort Erie and two days later defeating Canadian forces at Ridgeway, and again retreating to and holding Fort Erie. When 20,000 Canadian militia marched on the fort, the Fenians had already retreated over the border to a hero's welcome. Another attempt took place on 7 June when Fenians invaded Canada East (Québec) from Franklin, Vermont, occupying Pigeon Hill before retreating to the US. Sweeny was arrested 6 June 1866 by American authorities in his St. Albans hotel room but eventually released without trial.
Sweeny's plan for an invasion of Canada failed for many reasons: insufficient men, supplies and artillery; underestimation of the numbers of Canadian militia; the misguided assumption that most Canadians would welcome ‘liberation’ from Britain and the establishment of a Fenian republic; and the fact that the British consulate in Washington had received advance warning of the invasion. When he drew up the invasion plans, he expected $450,000 and 10,000 men, each with 200 rounds of ammunition, but had to make do with a much smaller force. Sweeny was blamed for the failed raids and forced to resign his position in the Brotherhood at the Troy, New York, convention in the autumn 1866. Leaving the Fenians, he rejoined the US army and commanded various posts in the southern department. He retired from active service 11 May 1870 with the rank of brigadier-general, and returned to his Long Island home. During his retirement he was involved in raising funds for the erection of a home at Austin, Texas, for disabled Confederate veterans. He died 10 April 1892 at his home and was buried with military honours. He married, first, Eleanor Swain Clark of Brooklyn, New York, and second, Eugenia Octavia Reagan of Augusta, Georgia. He had three sons and one daughter.