St. Mary's held firm initially thanks to some strong defensive work, but 'Belvo eventually broke their resistance 11 minutes in when a fine move across the opposition line was finished off in the right corner by second row Josh Eden-Whaitiri.
Former Leinster out-half Shaun Berne was wide of the mark with the subsequent conversion from a tight right-hand angle, but he made no mistake with his second kick at the posts just nine minutes later.
He split the posts from a three-point opportunity on this occasion, as 'Belvo opened up an eighth-point cushion by the end of the opening quarter.
St. Mary's were struggling to make impact in the 'Belvo half and when the Anglesea Road outfit introduced Leinster's Leo Auva'a for the injury-stricken Kieran O'Gorman on 24 minutes, their task became all the more difficult.
Full-back Gavin Dunne did have a chance to open his side's account a minutes later, but he was surprisingly off target from a kickable distance, and just shy of the half-hour mark, 'Belvo pounced for their second try of the day.
Right winger Mark Corballis, who was a late replacement for Steve Crosbie in a reshuffled 'Belvo back-line, applied the finishing touches to an excellent passage of play. With Berne also adding the bonuses, the visitors were threatening to pull away from their south Dublin rivals.
However, St. Mary's came to life during a frenetic end to the opening period. After a long spell of pressure in the 'Belvo 22, centre Robbie Hudson crossed the whitewash in devastating fashion for a much-needed converted try.
This ensured that the deficit had been reduced to eight points (15-7) just in time for the interval, which left the home side with a fighting chance upon the resumption.
The 2012 champions were hit with a sucker punch in the opening attack of the second half though, as winger Philip O'Dwyer ran through unopposed on the left flank for a superbly-executed score that was inevitably converted by Berne from the touchline.
There was a real danger that the concession of this try would damage the confidence of St. Mary's, but their challenge was instead galvanised.
Hudson duly secured his second try of the game five minutes into the half following a terrific kick through by promising out-half Cathal Marsh.
The returning Dunne easily slotted the routine conversion between the posts, and with 'Belvo hooker Cillian McDonald finding himself sent to the sin-bin as a result of Hudson's five-pointer, the momentum had suddenly swung in the direction of St. Mary's.
A coolly kicked 54th minute penalty by Dunne brought them into losing bonus point territory, and simultaneously set up a tantalising final quarter of action.
As the half wore on, St. Mary's sensed that a memorable comeback was on the cards, but despite the best efforts of the likes of Marsh, Hudson and Dunne, they were finding it difficult to break through the 'Belvo rearguard for a third try.
Old Belvedere, on the other hand, were far more incisive when they ventured into the opposition half. They finally copperfastened their victory - as well as the bonus point - when number 8 Jack Conan drove over in the right corner at the end of a sustained period of play deep inside the St. Mary's 22.
The character shown by St. Mary's on the day had to be commended, however. After much persistence they earned a consolation point when replacement Ian O'Neill powered through for a converted try seven minutes into injury-time.
Referee: David Wilkinson (IRFU)