Forrester signed a three-year contract with Scottish Premiership club Aberdeen in June 2018.
After a fantastic 2012 season, Forrester was called up to his first ever international squad when he was called up to Noel King’s Republic of Ireland under 21 squad on 25 January 2013 for a friendly against Netherlands under 21’s at Tallaght Stadium on 6 February. He came on in the 57th minute to make his first appearance on the international scene in a 3–0 win for the Republic of Ireland. On 8 March 2013, he was watched by Republic of Ireland manager Giovanni Trappatoni in Pats’ league opener, a 1–0 win against Drogheda United. Forrester retained his place in the Republic of Ireland under 21 squad for the Friendly game against Portugal under 21’s when Noel King made his squad announcement on 13 March 2013. He came on from the bench at half time in a 2–1 loss for the Republic of Ireland at Oriel Park.
Forrester received his first call-up to the senior Republic of Ireland squad on 11 March 2016 for Ireland’s friendlies against Switzerland and Slovakia.