Born in Castlebar, County Mayo, he moved to Dublin with his family in 1906 and was educated at the O’Connell Schools, North Richmond Street. He was a medical student at UCD at Easter 1916 and fought on the periphery of the GPO. He worked as a full-time organiser for the IRA from 1918, commanded the Second Southern Division, was captured in December 1920 but escaped in February 1921 without having revealed his true identity. Strongly anti-Treaty, he fought in the Four Courts and was among the last to vacate the building, directing the garrison's surrender. He was Director of Organisation and Acting Assistant Chief of Staff for the anti-Treaty forces and was captured during a raid on the O’Rahilly house in Ailesbury Road in November 1922. He was one of the last prisoners to be released in July 1924 after an imprisonment that had included a forty one day hunger strike and his election as Sinn Féin TD for North Dublin in the 1923 general election. At Frank Aiken’s invitation he accompanied him on the 1927 trip to the United States to raise funds to establish the Irish Press. Much of the rest of his life was taken up with travelling, journalism and broadcasting; and with recording the experiences of former colleagues in a series of interviews which form the O'Malley Notebooks. On Another Man’s Wound, his account of the War of Independence, was published in 1936; The Singing Flame, dealing with the Civil War, was published posthumously in 1978; and Raids and Rallies which originally appeared in the Sunday Press in 1955, was published in 1982. He married Helen Hooker in 1935.

The collection had been held for safekeeping in the Gilbert Library/Pearse Street Public Library for a number of years but was transferred to UCDA under the terms of the O’Malley Trust established in September 1974.