Gallagher was first elected as a TD in 1981. He has served nine Dáil terms since then representing Donegal constituencies. He was elected to Donegal County Council in 1979 and was elected as chairman of the council from 1985 to 1986. He has held various Minister of State positions including marine, gaeltache, environment, health and transport during his political career. He was elected as an MEP in 1994, 1999 and 2009.