General Sweeney was born in Burtonport, County Donegal in 1897. He was educated at St. Eunan's, Enda’s Dublin, U.C.D. and U.C.G. where he studied engineering. At St. Edna's he came under the wing of Pádraig Pearse. He joined the Volunteers in Dungloe in 1914 and transfered to the 4th battalion Dublin Brigade in September of that year. He was on the roof of the GPO during the Easter Rising, and was subsequently imprisoned in England and Wales.

He later rejoined E Company and during his summer holidays re-organised the Dungloe Company. In 1917, he was appointed commandant of the West Donegal Bat. In 1918 Commandant of No. 1 Brigade and in 1921 Commandant of the 1st Northern Division. After the truce, he was appointed O.C. of the Donegal Command (1923-March24). He was Sein Fein M.P. for West Donegal 1918-22 and he was T.D. from 1922-23.

During the army mutiny in March 1924, he was acting Chief of Staff of the Free State Army. 1924-1927 he was General Officer Commanding the Curragh Command, G.O.C. Western Command, and G.O.C. Curragh Command in that order. In October, 1928, he was appointed Adjutant-General. February 29 he was appointed Quartermaster- General. In June 1929, he became Chief of Staff.

He retired from the Army in 1940 where he became an inspector for Canada Life Assurance Co. until 1950 when he was appointed Area Officer with the Irish Red Cross Society for the twenty-six counties, and General Secretary of the organisation 1956 to 1962. He died in 1980, and is buried in the family plot at Dungloe in his beloved Donegal.