Malachy's dominated the first half with Liam Pettigrew and centre Oisín Kerlin showing strength under the Templeogue boards, putting the Belfast side up 40-25 at the break.
However, Templeogue were a transformed team in the third quarter, and a combination of some excellent work from captain Anton McDonnell and crucial baskets by Baolach Morrison and Sean Flood saw them just a point behind by its end, 54-53.
The exertions of the quarter took their toll, however. And although backed by terrific support on the day, Templeogue soon lost further ground.
The contribution of brothers Conor and Aiden Quinn proved instrumental for the Belfast boys, as Malachy's opened up a crucial seven-point lead with 10 minutes to go and from there were never headed. There was better news for Palmerstown Community College as they overcame Coláiste Colm Tullamore 68-52 to capture the U-19 'B' decider.
Inspired by MVP Cyril Kwan, Palmerstown forced a series of turnovers towards the end of the first half, with a hook shot from Brian Nyando seeing them 34-26 up at the break.
And while Palmerstown extended their lead to 14 shortly after the turnaround, a terrific outside shot from John Kavanagh combined with a free throw and subsequent lay-up from James Lalor left Tullamore seven points adrift heading into the final quarter, 48-41. Tullamore soon had that gap down to four points, but some fine defending, notably from Kwan, saw Palmerstown home in the end.