He was the youngest of five sons born to Paddy Joe Stephenson, former Chief Librarian of Dublin and a founder of the Old Dublin Society, who had fought in the 1916 Rising and had helped to restore Kilmainham Gaol. His elder brother was the renowned artist Desmond Stephenson. Sam married Bernadette Flood and they had two daughters Karen and Bronwyn and two sons Mark and Sam. His second marriage was to Caroline Sweetman, daughter of Barbara and Michael Sweetman, and they had two sons, Sebastian and Zachary.
Central Bank of Ireland Dame Street (1975) - It was built higher than planning permission allowed but this was rectified retrospectively. The matter was debated in the Oireachtas in 1974.
ESB Headquarters at Fitzwilliam Street (1976) - A block of Georgian houses was demolished to make way for this building. This destroyed the Georgian Mile in Dublin.
Dublin Corporation Offices at Wood Quay - Phase 1 (1976) - The remains of Viking Dublin were discovered during the construction of this building. Despite protests to save the site, construction went ahead. The buildings were known as The Bunkers because of their severe appearance.