William Aylmer (1778-1820) from Painstown, County Kildare, Ireland was a leader of the United Irishmen in the 1798 Rebellion against the British government. At the Battle of Ovidstown on 19 June 1798 he led a battle against British forces in which 200 insurgents died. Aylmer retreated into the inaccessible Bog of Allen and set up a defensive camp for over a month. Eventually he surrendered in return for a safe conduct abroad; effectively a form of exile.
Released from prison in 1802, he went into exile to Austria, where he served as an officer and noted swordsman in the Austrian Army, from which he was at one point detached to tutor British Dragoons in the art of swordsmanship.
After almost twenty years in Austria he returned to Ireland, and in 1819 sailed from Dublin to Venezuela with 200 officers and men to assist Simon Bolivar's independence struggle as commander of the Tenth Lancers and second in command of the Irish Legion under lieutenant-colonel Francisco Burdett O'Connor. He arrived in September 1819 on the island of Margarita off the coast of Venezuela, where lack of preparations caused severe hardships. Many of the volunteers died or returned to Ireland. He was wounded at the Battle of Rio Hacha on 25 May 1820 and died in Jamaica on 20 June 1820.
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- Seamus Cullen, Hermann Geissel (1998). Fugitive Warfare: 1798 in North Kildare. Lord Edward Fitzgerald 1798 Committee, with CRS Publications. ISBN 0-9533534-0-0.
- The Gentleman's Magazine. W. Pickering. 1846. p. 541.
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- Edmundo Murray. "Dictionary of Irish Latin American Biography: O'Connor, Francisco Burdett". Society for Irish Latin American Studies. Retrieved 2009-05-09.
- Moises Enrique Rodriguez (2006). Freedom's Mercenaries: British Volunteers in the Wars of Independence of Latin America. Hamilton Books. p. 164ff. ISBN 0-7618-3437-0.
- Brian McGinn. "St. Patrick's Day in Peru, 1824". Society for Irish Latin American Studies. Retrieved 2009-05-09.