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Born in Kilkenny in 1893 he was one of the 'driving forces' behind the anti-Treaty IRA in Dublin during the Irish Civil War. He was briefly the Officer Commanding of the IRA's Dublin Brigade and was interned in 1923. During this period he was also involved with the first Communist Party of Ireland.
Fitzpatrick was a full-time official of the Grocers' trade union and secretary of its social club at the Banba Hall in Dublin's Parnell Square. He also managed the Balalaika Ballroom and restaurant in the same area.
He was the central figure in IRA contacts with the Soviet Union during the late 1920s and in 1927, he attended the first International Congress of the Friends of Soviet Russia (FOSR) in Moscow. In 1928 he helped establish an Irish section of the FOSR. During 1929 he was involved in launching the Irish Labour Defence League and the Workers' Revolutionary Party of Ireland. He was also involved in Comhairle na Poblachta, a body set up the same year to heal the rift between the military and political anti-Treaty forces in Ireland. He visited the Soviet Union again in 1932.
Fitzpatrick chaired the 1933 IRA General Army Convention (GAC). At the 1934 GAC he disagreed with the call for a Republican Congress and remained within the IRA. His union was involved in a strike with O'Mara's Bacon Shops in late 1934 in which the IRA intervened violently. During 1935 he was involved in the IRA's intervention in the Dublin transport strike.
In 1936 he was an unsuccessful candidate for Cumann Poblachta na hÉireann, a political party set up by the IRA. Fitzpatrick succeeded Tom Barry as chief of staff in 1937, only to be ousted by Seán Russell at the 1938 GAC.
He was involved in the launch of the Clann na Poblachta political party in 1946 and was a member of its national executive. At the 1948 general election, he was elected as a TD for Dublin North-West, winning 2,395 votes (10.3%). At the 1951 general election, he received a meagre 458 votes (1.9%) and lost his seat.