Previous clubs: Wigan Athletic, Dundee United, Inverness CT, Northampton Town, Leicester City.
Mckay has been a prolific goalscorer in Scotland, excelling at Inverness CT and Dundee United.
The striker has netted just under 100 competitive goals since turning professional in 2007.
He joined Latics from Wigan Athletic on a season-long loan in July 2016.
Mckay has previously worked with manager Steve Robinson with the Northern Ireland national team and is one of Athletic’s key signings this summer.
He is a threat in the penalty area and has a keen eye for goal.