Following the Dunkirk evacuation in May 1940, Irwin was made a Companion of the Order of the Bath on 11 July 1940. From 28 October he commanded the 38th (Welsh) Infantry Division in Britain. His rank of major-general was made permanent on 29 March 1941 (with seniority backdated to 22 November 1940). On 7 November 1941 Irwin, promoted to the acting rank of lieutenant-general, commanded the XI Corps, which was based in East Anglia and had substantial responsibilities for the defence of Britain in the event of a German invasion.