Josh Eden-Whatiri and Stephen Crosbie were the heroes in Templeville Road as their respective try and conversion gave Old Belvedere a dramatic victory in a tight Division 1A encounter.
Mary's got the scoring under way when full-back Gavin Dunne split the posts with a penalty five minutes in. Belvo responded with a penalty of their own from Crosbie three minutes later, but Mary's rallied to reclaim the lead midway through the half, when winger Conor Hogan crossed over.
Dunne secured the bonuses and added to his tally with a second penalty, giving Hugh Hogan's charges a 10-point interval cushion, 13-3.
In the second period, penalties from Crosbie and Dunne meant Mary's still led. Belvo gave themselves a lifeline on 66 minutes when Jack Conan touched down. Eden-Whatiri then pounced in stoppage-time, and it was left to the reliable Crosbie to seal the comeback.
ST MARY'S COLLEGE -- G Dunne; C Lilly, M O'Driscoll, R Hudson, C Hogan; P Brophy, M D'Arcy; C McMahon, R Sweeney, A Lyons (T O'Reilly 73), C Ruddock, G Sexton (D Keller 72), K Haupt (K Sheahan 64), G Austin, K Croke.
OLD BELVEDERE -- D Riordan; J Glynn, J Kennedy, B Woods, C Murphy (D Symmonds 59); S Crosbie, F Lawlor (B O'Neill 21); P O'Regan (G Duffy 35), M Cooney (C McDonald 62), D Lavery (P O'Regan 58), J Eden-Whatiri, C Mallon (D Moore 43), J Conan, K O'Gorman, J Slattery.
REF -- S Gallagher (IRFU).