Having taken his MA in History at UCD, Gallagher joined the Civil Service applying for a position in the Department of External Affairs, as it was then called, starting in the job on 6 January 1969.
On Saturday, 16 August 1969, Gallagher, as weekend duty officer for the department, encountered a group of nationalist MPs from the North, including Paddy Devlin, who arrived at Iveagh House demanding to meet Taoiseach Jack Lynch to obtain arms for the beleaguered Catholics of the Falls Road in Belfast.