This was St. Mary's fifth straight victory and now gives them both momentum and confidence going into their heavyweight showdown at Dubarry Park on May 8 against the in-form Cork Constitution, who beat Dolphin 31-18 in the other semi-final.
St. Mary's, despite being on the defensive for most of the first half, led Old Belvedere 9-3 at the break.
To compound 'Belvo's woes, their player-coach and captain Andy Dunne had gone off injured just before half-time, forcing director of rugby Phil Werahiko to make some big changes to the back-line. Conal Keane moved from centre to out-half and replacement Kevin Barden slotted into midfield.
One would have expected St. Mary's to take control of the game at the start of the second half, yet they almost let the game slip from their grasp.
Within ten minutes, 'Belvo had mustered two penalties from Keane and a Richie Leyden try, which was converted by Keane, to give them a deserved 16-9 lead.
The final quarter was a nail-biting period, with the lead going back and forth but ultimately St. Mary's got the edge by scoring two tries, one from centre Mark Sexton and the clincher from Grissing.
Speaking after the match, Sexton expressed his contentment when saying: "An unbelievable win. 'Belvo really put it up to us, we were probably 50-50 throughout the whole game and fortunately we came into our own near the last 20 minutes and really started playing.
"For my try, the team really put in the work. I think we went through six or seven phases and I ended up going over but I think it could have been anyone of the fifteen."
Player-coach Shaun McCarthy was very much a crucial figure in the victory, providing the scoring pass for Sexton's try and essentially adopting a fast-paced game-plan which gave St. Mary's the edge in the final quarter.
The Australian out-half praised the team for reacting well to a tough situation. He said: "We thought we had the strong hold at half-time going in 9-3 but it just goes to show you can't fall asleep.
"You can't take anything for granted and they come back 16-9 and then we had to pull it out of the bag.
"But fair play to our guys, we just knew we had to stay composed and pick up the tempo towards the end and it worked."
St. Mary's director of rugby Peter Smyth will be hoping that his players can achieve what he did ten years ago in 2000 by lifting an AIB League title for the Saints.
Offering his take on the encounter, he said: "I'm just absolutely delighted. It's just relief more than anything. Credit to the guys, they never stopped believing and they got over the line at the end.
"Obviously, I'm a little bit happier because we've come out on the right side but we've had our close shaves over the year when we've fallen short as well so we've got to enjoy now.
"But we said coming into this game no matter, we've kept the ball in hand all year and we tried to do that today. Luckily enough the RTE cameras were here and hopefully the whole country saw a really good game."
'Belvo, who have been promoted to Division 1A, were looking to get revenge after they were narrowly beaten by St. Marys 9-3 in the final of the Leinster Senior League Cup earlier this year, however they will now have to wait until next season.
Deservedly, Smyth offered due recognition to the men from Anglesea Road after Saturday's semi.
"It goes without saying, 'Belvo could have won today and they could have gone on and won the final," he admitted.
"They're a tremendous team and we've been lucky against them twice this year, so they're going to be very difficult opponents in Division 1A next season."
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