Growing up, McStay's mother took him to football training and made sure he got to his matches, which is why he cites her as the largest influence on his career.
McStay is the son of Celtic FC legend Paul McStay who made over 650 league appearances for the club between 1981 and 1997. He also has two great unclesJimmy and Willie who captained Celtic, and an uncle who also represented Celtic, Ray. His family resides in Australia.