Cahill studied at University College Dublin and the Architectural Association, London. Prior to returning to Ireland in 1979. he worked with the London Borough of Camden, Levitt Bernstein Associates and the Royal College of Art Project Office. Established Gerry Cahill Architects (GCA)in 1984 and has specialised in social housing and in building conservation and restoration. Active in many areas of community development, he has produced several publications on urban renewal in Ireland, including Back to the Street (1980).