In 1915 he joined the 2nd battalion of the Dublin Brigade of the Irish Volunteers at age 14. On Easter Monday 1916 he assembled in Stephens Green and was supposed to march over to Jacobs Factory where he would remain for the rest of the week. He was told to go home and started to cry. On his way home he ran into his section commander who asked him what was wrong. When Byrne explained what had happened he was brought back to the Green.
He spent the remainder of the week in Jacobs Factory under the command of Thomas McDonagh. When the rebels surrendered he escaped to his home but was arrested a week later and taken to Richmond Barracks.
On the morning of the day of Bloody Sunday (1920) Byrne assassinated Lieutenant Aimes and Lieutenant Bennett in their home. As he and his men were fleeing they got into a shoot out with British agents.