Born in Scotland, he represented the Republic of Ireland at international level, making two appearances for the team in 1967.
Gallagher first played football for St. John's (Gorbals) Boys' Guild in 1952 and Holyrood Senior Secondary the following year.
[3] John Murphy, his secondary school physical education teacher who also served as an announcer at Parkhead on match days, recommended him to Celtic.
[5] He then made his full debut on 6 March the following year, when Jock Stein signed him as to a professional contract that same day.
[3] Gallagher took the corner kick that resulted in Billy McNeill's winning goal in the final of the 1965 Scottish Cup.
[3] He scored 40 goals in 95 games for the club,[7] and won the Division Two championship in 1971–72,[8] before retiring from professional football in April 1973.