Harold O'Sullivan was a former general secretary of the Local Government and Public Services Union (predecessor of Impact) and president of Ictu, the Irish Congress of Trade Unions. Prior to his trade union career, he served as an officer in the Cavalry Corps, and when he retired turned to writing history. He was the author of Irish Historic Towns Atlas, no. 16, Dundalk (2006) and joint author, with Raymond Gillespie, of Irish Historic Towns Atlas, no. 23, Carlingford (2011).