Replacements: Paddy Hogan, Adam Howard, Josh Eden-Whaitiri, Fergal Lawlor, Mark Corballis.
Replacements: Brian Byrne, Tom Byrne, Keith Donoghue, Peter du Toit, Niall Myron.
Michael McGrath was in top form for Andy Wood's side last year and it took just three minutes of the new league campaign for the winger to open his try-scoring count, but former Trinity out-half David Joyce was wayward with the conversion.
Barely a minute later Clontarf were in again, with centre Evan Ryan finished off the move after some immense forward play. Joyce added the conversion and a penalty shortly afterwards to put Clontarf cruising at 15-0.
But Old Belvedere bounced back and were rewarded when former Leinster back Shaun Berne, playing at out-half, went in under the posts for an 18th minute try which Glynn converted.
Clontarf looked to have restored order when Joyce converted a penalty five minutes later but it could have been more had they made the best of an interception. Glynn responded for Old Belvedere with a penalty of his own for 18-10.
Then, with five minutes left in the first half, 'Belvo broke through once again, with centre John Kennedy touching down in the corner for an unconverted effort, cutting the gap to 18-15 for half-time.
The second period failed to live up to the thrilling attacking rugby of the first, with handling errors and penalties the order of the day.
Ten minutes in, Glynn had completed a remarkable turnaround with two penalties to give 'Belvo a slender three-point advantage. There was an opportunity to stretch their sudden lead but, for once, his effort was off target.
Clontarf clawed their way back with a Darragh Fitzpatrick penalty to level the game at 21-all on the hour mark, and should have made the most of a three-on-one overlap but centre Killian Lett was stopped short.
With the game in the balance, Clontarf replacement Tom Byrne was sin-binned for a tackle on Glynn who looked to gather his own up-and-under. Glynn had a 30-metre penalty to send 'Belvo into a three-point lead and he duly obliged, but there was still time for a dramatic finish.
Try scorer Kennedy was penalised at the ruck but saw the 'Tarf penalty attempt sail wide of the uprights with five minutes to go.
In the final minute the north Dubliners looked to have snatched victory from the jaws of defeat but referee Eddie Hogan O'Connell ruled that the ball was knocked on as 'Tarf crossed the whitewash.
'Belvo were left to celebrate their first opening day win in the league since 2010, while Clontarf will hope to bounce back against Garryowen next weekend.
Referee: Eddie Hogan O'Connell (IRFU)