James O'Donovan (Irish: Séamus Ó Donnabháin; 3 November 1896 – 4 June 1979), also known as Seamus or Jim O'Donovan, was a leading volunteer in the Irish Republican Army. He fought in the Irish War of Independence and then on the Anti-Treaty side during the Irish Civil War. He was an explosives expert and was imprisoned a number of times. He is best known for his contacts with the Abwehr military intelligence of Nazi Germany.
A native of County Roscommon, he was an explosives expert and reputedly invented the "Irish War Flour" (named after the flour sacks in which it was smuggled into Dublin aboard ships) and "Irish Cheddar" devices. He subsequently became IRA Director of Chemicals in 1921. During the Irish War of Independence he was imprisoned in Mountjoy and Kilmainham prisons and later interned in Newbridge, County Kildare.
He opposed the Anglo-Irish Treaty and fought in the Irish Civil War.
In 1930 he became manager at ESB headquarters in Dublin.
In august 1938, at the request of IRA chief of staff Seán Russell, he wrote the S-Plan, a bombing campaign targeting England. During this time O'donovan and Russell were the only GHQ members of the old IRA still in the organisation.
In his unpublished memoirs he wrote that he "conducted the entire training of cadre units, was responsible for all but locally-derived intelligence, carried out small pieces of research and, in general, controlled the whole explosives and munitions end" of s-plan.
As "Agent V-Held", he visited Germany three times in 1939 on behalf of the IRA.
On 28 February he negotiated an arms and radio equipment delivery at the Abwehrstelle in Hamburg. On 26 April he concluded a new arms deal with the Abwehrstelle and established with the help of a Breton a secret courier connection to Ireland via France. On 23 August, O'Donovan received the last instructions for the event of war.