Byrne, William (c.1775–1799), United Irishman, was youngest son of Garret Byrne of Ballymanus, Co. Wicklow. In late 1797 he joined the Wicklow town yeoman cavalry, from which he was ejected (February 1798) for refusing to swear a test oath requiring him to abjure allegiance to the United Irishmen. Byrne was almost certainly then a member of the United Irishmen, and once the rebellion broke out he replaced his absent brother Garret (qv), a senior figure in the movement, at the head of the south Wicklow rebels. Another brother, Edward ‘Ned’ Byrne, was also a conspirator of some stature. William Byrne took command of the ‘Ballymanus division’ at Mount Pleasant hill (Tinahely) on 8 June and led them during several key engagements in north Wexford and south Wicklow. He distinguished himself at the battle of Arklow (9 June) and at Vinegar Hill (21 June), after which he relinquished command to Garret Byrne and retired from the rebellion. He received a conditional pardon in late June from Wexford's yeomanry brigade-major, Fitzgerald, and some months later obtained permission to raise a regiment to campaign against the French in Egypt. He moved to Dublin city to escape the scrutiny and threat posed by Wicklow's ultra-loyalists, who succeeded in having him arrested at his lodgings with Mrs Passmore at 6 Francis St. (14 May 1799). He was sent to Wicklow gaol to await court martial on several serious charges including murder and defection from the yeomanry, which both negated the validity of his pardon. The trial began on 24 June and ended, after an adjournment, on 2 July, when he was sentenced to death for acting as a rebel leader during the insurrection. He was executed (24 September 1799) at Gallows Hill near Wicklow town.
Byrne died unmarried. His youth, social status, and heroic activities as a rebel leader endeared him to many contemporaries and he was eulogized in the popular ballad ‘Billy Byrne of Ballymanus’ shortly after his execution. He was one of four Wicklow leaders commemorated on Wicklow's main 1798 memorial in the county town, popularly but erroneously known as the ‘Billy Byrne’ monument since 1899.