At the age of 17 Byrne was asked to join the government Yeomanry He choose instead to join the Society of United Irish which, in defiance of the British Crown and the Protestant Ascendancy, was determined to achieve an independent and representative government for Ireland. He participated in preparations in Wexford for the 1798 Rebellion, and at the age of 18 fought at the Battle of Tubberneeing on 4 June and, in command of a division of pikemen, in the attack on Arklow (9 June) in which the rebel leader Father John Murphy was killed. In the face of a general rout, he led a rebel charge in the Battle of Vinegar Hill (21 June).