Phil Werahiko's side enjoyed the better of play in the first half but simply could not muster an attack to get them over the try-line.
Spurred on by their athletic, ball-carrying back row of number 8 Leo Auva'a and flankers Jason Risdon and Richie Leyden, the black and whites employed effective pick-and-go tactics, forcing St. Mary's to defend with everything they had.
Conal Keane managed to slot in nicely for Dunne at out-half after the break, scoring two penalties and converting Leyden's well-executed try to turn the game around and give 'Belvo a 16-9 lead.
After this, the lead was to change hands three times before the final whistle. Firstly, St. Mary's brought the scoreline to 19-16 after centre Mark Sexton's try along with a magnificent drop goal from man-of-the-match Gavin Dunne.
Then Leinster scrum half Chris Keane snuck through a hole in the St. Mary's defence to touch and 'Belvo took a four-point lead.
At the end of the day though, it was centre Stephen Grissing who became the Templeville Road hero when went over in the corner to score the match-winning try.
After the match, Werahiko talked of how Dunne's injury had changed the game, saying: "Andy is obviously experienced, he has a lot of composure. Obviously he carries a game-plan in his head and when that game-plan comes off the park, you always have players on the bench who can come on and that's what happened today.
"I thought Conal Keane who played twelve, I thought he did an exceptional job, probably one of his best games for the club.
"So, to replace a player like Andy and he came in to out-half after half-time get us back level at 9-9 and then he directed the game and we managed to get ourselves ahead, so I thought he did an outstanding job."
'Belvo had to make do without star centre Fergus McFadden who was on Leinster duty in the Magners League on Friday night and Eoin O'Malley, who was on the bench against the Ospreys, was also not included in the squad.
Although 'Belvo were still a capable force without these players, they could have provided that bit of attacking skill that was needed to breach the St. Mary's try-line in the first half.
Werahiko acknowledged the problems of having contracted players in the side, adding: "When you've got these players, you can have the tendency to rely on them.
"But their first priority is to the provinces so you can't really rely on them. You have to try and build a side."
He rightly pointed to St. Mary's as a club which has built up an excellent team without having to rely on contracted players.
"I think St. Mary's are one of the first clubs that have gone about their business building a squad without contracted players.
"Ten years ago when they won the league, they lost nearly a team of guys who went professional and they felt themselves going back to the second division in twelve months.
"Since then, they've built up and they deservedly made their way back up to the top using good players that aren't involved in professional rugby."
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