Richmond Park witnessed a 1-1 draw between Pat's and Derry City before Saints' captain Conor Kenna lifted the Airtricity League trophy as fans celebrated the league trophy's return to Inchicore.
Derry had taken a first half lead through Barry McNamee after he latched onto a Paul Curran pass to slot the ball past Saints' keeper Brendan Clarke.
John Russell and Conan Byrne replaced Sean Gannon and Chris Forrester as Liam Buckley tried to turn things around, but it was the manager's third change that proved decisive.
Anto Flood, who replaced Saints' striker Christy Fagan on 63 minutes, picked the ball on the left, before cutting inside and finding the net with a powerful strike on 81 minutes.
The goal only served to increase the party atmosphere in Inchicore as the many Saints present saw skipper Kenna lift the Airtricity League title after the game to begin the celebrations long into the night.
St Pat's: Clarke, O'Brien, Kenna, Price, Bermingham; McFaul, Brennan, Gannon (Russell 45); Kelly; Forrester (Byrne 45); Fagan (Flood 63).
Subs Not Used: Fitzgerald, Kavangh, Browne, Gilmartin.