Tomás Ó Deirg was born on 26 November 1897 in County Mayo. He was educated locally and at University College Galway. During his time in college he organised a corps of the Irish Volunteers. After the 1916 Easter Rising he was arrested and imprisoned. After his release he graduated from college and became headmaster in a technical college in Mayo. During the War of Independence Ó Deirg was interned at the Curragh. While there he was elected Sinn Féin MP for Mayo.
Ó Deirg took the republican side after the Irish Civil War. He was later captured by the Irish Free State army. In 1927 he was elected to Dáil Éireann as a Fianna Fáil TD. In Eamon de Valera's first government in 1932 Ó Deirg was appointed Minister for Education. Between 1939 and 1943 he served as Minister for Lands. He was re-appointed to Education in 1943 until 1948. Between 1951 and 1954 he became Minister for Lands again.
Tomás Ó Deirg died on 19 November 1956 in Dublin.