Peadar O'Donnell was born in County Donegal, Ireland in 1893. He attended St Patrick's College, Dublin, where he trained as a teacher. He taught on Arranmore Island before spending time in Scotland.
O'Donnell joined the Irish Republican Army (IRA) during the War of Independence. He was imprisoned in Mountjoy Gaol and went on hunger-strike for 41 days.
In 1924 he became a member of the Executive and Army Council of the IRA. His attempts at persuading the IRA to become a socialist organization ended in failure.
O'Donnell remained active in politics and helped establish the Workers' Revolutionary Party and edited its newspaper, The Workers' Voice. A founder member of the Republican Congress, O'Donnell, was also a leading opponent of Eoin O'Duffy and the Blue Shirts.
On the outbreak of the Spanish Civil War O'Donnell urged the formation of volunteer regiments to support the Popular Front government. O'Donnell and Frank Ryan established the Connolly Column (named after James Connolly) and in December 1936, Ryan and eighty volunteers left Dublin for Spain. The majority came from the Free State but there were also a group of socialists from Belfast. O'Donnell also went to Spain and later published Salud! An Irishman in Spain (1937).
After the Second World War O'Donnell edited the Irish literary journel, The Bell (1946-54). Other books by O'Donnell include The Big Windows (1955) and Proud Island (1975). O'Donnell also published two volumes of autobiography, The Gates Flew Open (1932) and There Will be Another Day (1963).
Peadar O'Donnell died in 1986.