Replacements: Killian O'Neill, Adam Howard, Leo Auva'a, Fergal Lawlor, Mark Corballis.
Replacements: Scott Flanagan, Arthur Lyons, Mark Fallon, Robbie Glynn, Marcus O'Driscoll.
Skipper Daniel Riordan soon replied with his sixth try of the current season to bring 'Belvo level, although replacement Fergal Lawlor was unable to add the winning conversion for the second-placed side.
St. Mary's really burst out of the blocks in this breathless rain-hit encounter, hitting the home side for two converted tries as they sped into a 14-0 lead.
Hard-running and quick recycling led to the opening try for centre Robbie Hudson as the Belvedere defence gave way in the tenth minute, with the conversion added by full-back Gavin Dunne.
Hudson's brother, Leinster-capped winger Darren, played his part in the build-up to the second try. He tore up the touchline and almost made the try-line from 50 metres out.
'Belvo coughed up a close-in penalty, St. Mary's captain Kevin Sheahan smelt blood and from the resulting lineout and maul, tighthead prop Brian McGovern muscled over for a well-worked try.
Dunne knocked over an excellent conversion, but the injury-enforced withdrawal of Darren Hudson was a setback for the visitors - despite his replacement Marcus O'Driscoll doing well.
'Belvo came more into the game in the second quarter and showed the sort of form that has them at the business end of the league table.
Paul Cunningham's men got off the mark after 32 minutes, advancing from a penalty and a well-won lineout as they built through the phases for pacy left winger Richard Dunne to go over in the corner. Crosbie did very well to land the conversion for 14-7.
The young out-half was wide with a subsequent penalty attempt, but he shaved three more points off the St. Mary's lead with a successful penalty six minutes into the second half.
The momentum was with 'Belvo suddenly as, in the 53rd minute, their in-form number 8 Jack Conan picked from a scrum and crashed over for a try which Crosbie converted.
This came with St. Mary's lock Ciaran Ruddock in the sin-bin, however Leinster 'A' ace Cathal Marsh got the visitors back on level terms with a penalty approaching the hour mark.
It was St. Mary's turn to force the issue as they got their attack in gear again. After a penalty miss, they quickly pressed again and the alert D'Arcy grasped his opportunity to score from five metres out.
His try went unconverted, leaving five points between the sides with little over a quarter of an hour remaining.
The tit-for-tat nature of the game continued when Belvedere gained good yardage, using the boot to good effect, and St. Mary's were caught short of numbers out wide as the wily Riordan pounced to score in the corner.
In a nail-biting finish, Lawlor missed his difficult kick from wide out and St. Mary's threatened to conjure up a match-winning score at the other end before referee Peter Fitzgibbon brought this cracking contest to a close.
Referee: Peter Fitzgibbon (IRFU)