Ahern's father, Eamon Ahern (1882–1953), was a member of Cork County Council from 1919 to 1953. He was a member of the IRA and the IRB. On Eamons' death, Liam was co-opted onto the Council. Ahern's uncle, John Dinneen, was also a member of the Dáil between 1922 and 1927. Liam's uncle, Maurice Ahern (1899–1950), was captain of the East Cork 4th Battallion Number One Brigade of the IRA. He led to Cork, in 1916, members of the Irish Volunteers from Dungourney, Clonmult and Ballynoe, under arms, and in the subsequent struggle was interned in Wakefield, Wandsworth and Frongoch prisons. Later, in Cork Prison, Maurice Ahern was the first man to go on hunger strike.
Liams' son Michael Ahern was a Fianna Fáil TD for Cork East from 1982 to 2010. Another son, auctioneer Maurice Ahern was a long standing Fianna Fáil councillor for the Midleton electoral area of Cork County Council from 1979 to 2009.
At the 1973 general election, he again contested Cork North–East, where he came fourth out of the four successful candidates, unseating the sitting Fianna Fáil TDSeán Brosnan. A Fine Gael–Labour Party coalition government under TaoiseachLiam Cosgrave took power at that election, ending a sixteen-year period of Fianna Fáil government, and Ahern found himself in opposition for the first time in his career in the Oireachtas. He caused a minor sensation during the Dáil debate on the seizure of IRA arms on the MV Claudia in May 1973 when he shouted out, "More guns we want, bags of guns."
He died in July 1974 at the age of 58, triggering a by-election on 13 November 1974 in which his seat was retained for Fianna Fáil by Seán Brosnan.