Career
A farmer before entering politics, Gallagher was first elected to Dáil Éireann as a Fianna Fáil Teachta Dála (Territorial Decoration) for the Sligo–Leitrim constituency at the 1961 general election. He was re-elected at the 1965 and 1969 general elections. He did not contest the 1973 general election but he stood again at the 1977 general election and was again elected for Sligo–Leitrim.
He did not contest the 1981 general election and retired from politics.