Irwin's record of service during the war was outstanding. He saw action on the Western Front throughout the conflict, initially serving with his battalion, then part of the 12th Brigade of the 4th Division, in most of the major battles of 1914 and 1915, before serving in the final two years as the Commanding Officer (CO) of the 1st and 2nd Battalions of the Lincolnshire Regiment and the 8th Battalion of the Leicestershire Regiment. Still a second lieutenant upon war's outbreak, Irwin advanced quickly in rank, being promoted to lieutenant on 24 September 1914, temporary captain on 9 May 1915, captain on 7 November 1915, acting major on 10 March 1917, and acting lieutenant colonel on 27 April 1917, before reverting to acting major on 1 June 1917. He was again made an acting lieutenant colonel on 23 August 1917. He was promoted to temporary lieutenant colonel on 6 April 1918, and again temporary lieutenant colonel on 1 July 1918.