Stynes played Gaelic football for the McCracken's club on the Northside, then transferred to the elite O'Tooles club in February 1922. He was an 'outstanding' forward with 'rare qualities'. He took the anti-Treaty side during the Irish Civil War, but managed to play several games for the senior Dublin county football team while "on the run" from the Irish Free State authorities. He fought in the O'Connell Street area during the Battle of Dublin, evading capture during the evacuation of the Hammam Hotel. An attempt to tunnel into Mountjoy Prison from an adjoining house ended when the house was raided while Stynes was absent. He missed Dublin's win in the 1922 All-Ireland final (played 7 October 1923) as by then he was interned in the Curragh Camp. Major General Tom Ennis of the National Army, a former teammate of Stynes who took the pro-Treaty side, secured Stynes' availability for subsequent Dublin matches, and he got a winner's medal for the 1923 final (played 28 September 1924), in which he scored the final two points. He missed the 1924 final (played 26 April 1925) after being suspended by the GAA for breaching its ban on playing "foreign" games by playing soccer, a game he had learnt while in the Curragh.