McStay was player/manager at
Sligo Rovers from 1992 to 1994 and he led them to a historic treble in 1993–94, when they won the
First Division,
First Division Shield and the
FAI Cup. In 1994, the
Scot left Sligo just before the start of the new season after being approached by Celtic to become their youth team coach. McStay was appointed the head coach of
Celtic Reserves in January 2007, and worked in that role at the club until July 2009, when he left to take up the role as manager of
Újpest FC in
Budapest, Hungary. His assistant at Celtic and at Újpest was
Joe McBride. McStay resigned as Head Coach of Újpest in April 2010, and was replaced by
Géza Mészöly. McStay returned to Celtic in April 2010 to take charge of the reserve side again after leaving Újpest, before departing 2 months later. On 25 November 2010, McStay was appointed manager of
Ross County. McStay left the club by mutual consent after less than three months as manager. McStay had taken charge of only nine games, none of which were won, during that time. McStay was appointed assistant manager of
Stockport County in July 2011 by manager
Dietmar Hamann. After Hamann's departure, in November, new manager
Jim Gannon kept McStay on as his assistant. McStay took over as manager of
Northern League Division One team
Celtic Nation in September 2013. McStay led his side to their first trophy as Celtic Nation on 29 April 2014, defeating
Aspatria 3–0 in the Cumberland Cup Final. McStay went on to lead the side to a second-place finish in the league, and went undefeated in their final 14 league games of the season. However, this was not enough to gain promotion to the
Northern Premier League. Over the following weeks, a financial review was carried out at the club which resulted in many of their players leaving. Matters reached a head in July 2014 when owner Frank Lynch sold the club on to former chairman
Steve Skinner. Amidst the turmoil at the club, McStay resigned. == Personal life ==