Methody came from five points down to defeat their south Belfast rivals at Ravenhill.
Josh Atkinson put Inst ahead when he crossed the line for the opening try of the game after 15 minutes although Nathan Brown missed the conversion.
Atkinson had to go off with an injury and then Methody out-half Josh Bingham touched down to level the scores after a spell of intense pressure.
Bingham missed the conversion and then failed with a drop goal attempt leaving the game all square at the break.
Conor Kelly scored Methody's second try shortly after the restart and Bingham added the extra points from the conversion.
The Methody number 10 then sent over a penalty to make it 15-5 and leave Inst with an impossible task.
Bingham then added the coup de grace with a late unconverted touchdown as his side lifted the famous trophy for the 35th time.