Mary's opened the scoring in the 18th minute when centre Ray Crotty slotted a penalty in front of the posts.
Just before the break, Conor Allen set off on a stunning sprint for Terenure which full back Stephen Walsh eventually stopped, but in the follow up, Allen's centre partner Darragh Newe was thwarted by Niall Sheehy's bravery, but openside Keith Kane drove over from barely a metre out for a 5-3 interval lead.
St Mary's were not too pleased to be behind and wing Sheehy made that point clear when latching onto Shane Timmons's chip to make a dash for the line. Crotty's conversion made it 10-5.
Crotty then made the initial burst for full back Stephen Walsh to weave his way into the 22 and out-half Conor Breen dropped back into the pocket to slice through the posts with a drop goal midway through the half.