Two-try hero Alan Gaughan paid credit to the Garryowen forwards for putting the ball up their jumpers to help secure this triumph.
The Limerick side were reduced to 13 men for the first 10 minutes of the second half when Declan Lavery and Corey Hircock were sin-binned.
Ballymena led 6-5 at the break but Garryowen dominated thereafter and a powerful double from Gaughan sealed the victory.
"It was hard-going in at half-time with 13 men but we knew we had to stick together and put the ball up the jumper for a while," Gaughan said.
"The forwards have worked so hard all year on their scrums and mauls and they did really well today."
Garryowen went 5-3 ahead after 21 minutes when Neil Cronin, younger brother of Leinster's Sean, charged down a box kick and touched down.
Luke Marshall scored two penalties for Ballymena but Gaughan's brace settled the contest.
There was also a late try for Ronan O'Mahony as he collected a bouncing ball following a Declan Cusack chip.
Garryowen captain Paul Neville, in his final game for the club, lifted the trophy at Templeville Road.
Garryowen -- L Burke; A Gaughan (J Gavin 67), C Hircock, D Cusack (J McKierney 75), R O'Mahony; F Lawlor, N Cronin; JP Cooney, J Rael (A McMahon 72), D Lavery; F McKenna (C Hartigan 67), D Sherry; P Neville (S Buckley 46), K O'Gorman (A Kavanagh 50), L Viapulu.
ballymena -- R McBurney (J Smith 70); A Frazer (I Vance 70), R Andrew, L Marshall, C Fitzpatrick; M Irwin, B McIlroy; C Black, John Andrew (A Warrick 11), S Shawe; (C Carey 70), D O'Kane, Joel Andrew; S Mulholland (A Hamilton 19), P Pritchard, R Hackney.
REF -- Eanna Falvey