He was later a prominent officer of Cork's No. 1 Brigade of the
Irish Republican Army during the
Irish War of Independence. On 3 September 1917 he led a successful raid for arms at
Cork Grammar School, a school for the children of the
Anglo-Irish gentry, which maintained its own armoury for training potential
British Army officers. O'Donovan acquired the nickname "Sandow" around this time because of his resemblance to famous
German bodybuilder Eugen Sandow. O'Donovan led or participated in many daring raids against British forces in
County Cork, including the capture of
Blarney's
Royal Irish Constabulary barracks on 1 June 1920. O'Donovan was also in command of the six-man IRA unit that carried out the assassination of RIC District Inspector
Gerald Bryce Ferguson Smyth at the Cork and County
social club on the evening of 17 July 1920. Some weeks earlier Smyth had gained notoriety when RIC Constables in
Listowel,
County Kerry mutinied rather than listen to his orders to "
shoot to kill" all persons who were suspected of
Irish republicanism. For this reason, O'Donovan is alleged to have called out before the hit team opened fire, "Colonel, were not your orders to shoot on sight? Well you are in sight now, so prepare." According to historians Tom Mahon and James Gillogly, "Smyth was the most senior police officer killed in the conflict." Under the command of
Seán O'Hegarty, O'Donovan and others organised the
Coolavokig Ambush near
Macroom in February 1921. He was also involved in the seizure of a large cache of guns and ammunition from the British
Royal Navy tender,
Upnor, off the coast of County Cork, after first commandeering a smaller vessel at
Queenstown. According to historians Tom Mahon and James Gillogly, "The IRA operatives seized so much arms and ammunition that they're reported to have needed 200 lorries to cart it all away." ==Civil War==