 After the outbreak of the War Iremonger was with the Grand Fleet and at the Battle of Jutland, on 31 May 1916, being the senior Marine officer in HMS Valiant of the 5th Battle Squadron.Early in 1917 Iremonger joined the RMA Heavy Siege Train operating in the Belgian coastal sector.In June 1917 he was promoted to major and in the following March became a ‘group’ commander and his guns fired in support of the Zeebrugge Raid.