Gallagher was born in Lewisham, London to Irish parents. An enthusiastic Arsenal football supporter, he was introduced to rugby union at his childhood school, St. Joseph's Academy, Lewisham, where he was required by the headmaster to join the team.
Rugby Union
Invited to play in Wellington, New Zealand, for the Oriental Rongotai club, Gallagher went on to play for the Wellington provincial team. It was at Wellington that he moved from the position of centre to fullback, a role at which he quickly excelled through a combination of intelligent decision-making, courage and, above all, speed. He was then chosen for the New Zealand national team, the All Blacks, for whom he played 41 games. Gallagher began with four outings in France in 1986, and continued through the 1987 season, cementing his test match starting spot and winning the inaugural Rugby World Cup that year in Auckland, New Zealand. In 1990 he was named the International Rugby Player of the Year. He did not suffer a loss while representing New Zealand, and is considered by many sources, including the New Zealand Herald and the Telegraph newspapers, to be one of the greatest fullbacks and All Blacks of all time.
Rugby League
In 1990, Gallagher was one of the first of a succession of All Blacks to switch codes to rugby league, following his understudy Matthew Ridge, who signed with the NSWRL's Manly-Warringah Sea Eagles only a week before. Other All Blacks who followed soon after were Frano Botica, John Schuster and two years later Va'aiga Tuigamala. He was initially signed by David Ward for Leeds RLFC. While he was considered one of the fastest players over 30m when he switched to league, and this was put to good early use by Leeds, though he was targeted by opposition players in defense and was badly injured in an alleged spear tackle. He may also have found the tactical differences between the two codes difficult to adjust to, particularly in his favoured full-back position and the move was unsuccessful. Perhaps his finest moment for Leeds came in a match against Australia during the Australian teams 1990 Kangaroo tour when he scored the opening try of the game at Headingley. Unfortunately though, Gallagher's defense came into question later in that game when giant Kangaroo second rower Paul Sironen scored two tries, both times running over the top of him. Gallagher found greater success in the less rarefied atmosphere of playing for the club then known as, where his reliable kicking and astute play made a significant contribution over several seasons.
Later career
Gallagher continued to play rugby union semi-professionally at Blackheath, but he also took up teaching, becoming headteacher at Colfe's Preparatory School in south-east London. In 1996 he represented Ireland 'A', playing in the centre.
