William Aylmer (1778–1820) was an Irish military officer and member of United Irishmen who participated in the Irish Rebellion of 1798. On 19 June 1798, Aylmer fought in the Battle of Ovidstown against British Crown forces, which resulted in a defeat for the United Irishmen. Aylmer retreated into the inaccessible Bog of Allen and established a defensive camp, which he maintained for over a month. Eventually, he surrendered to the British 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 Imperial Austrian Army, from which he was at one point detached to tutor the British Army's dragoons in the art of swordsmanship.