Clontarf's back-to-back losses to UCD and Lansdowne has opened the door for 'Belvo to reclaim the title they last secured in 2011.
With 23 points separating them and Dolphin in the Division 1A table before kick-off, they were favourites to return to the summit.
Yet, Dolphin also had good reason to produce their best form in the capital as they needed one more win to be certain of their Division 1A status.
With that in mind, it came as little surprise that the visitors made such an impressive start to proceedings.
Their young out-half Rory Scannell missed an awkward penalty opportunity on the left wing just two minutes in but Dolphin broke the deadlock three minutes later.
The move for the opening try was instigated by a strong drive on the right and hooker Niall Scannell, who lined out alongside his brother Rory in the Munster 'A' team's recent British & Irish Cup loss to Leinster 'A', powered over impressively in spite of the best efforts of the 'Belvo defence.
Rory Scannell converted and Old Belvedere's initial reaction to the concession of seven points was positive,.
Full-back Glynn, who moved from his customary role on the wing in the absence of captain Daniel Riordan, had an excellent opportunity to open his side's account on ten minutes.
His kick into a tricky breeze drifted well short of the mark however, and when referee Peter Fitzgibbon awarded a 35-metre penalty to Dolphin 14 minutes in, Rory Scannell split the posts emphatically.
Because of the significance of the game, there was a certain degree of nervousness about the 'Belvo play, but a successful three-pointer from close range by Glynn did help to ease the pressure on Paul Cunningham's men.
Indeed, as the minutes passed by, 'Belvo were making more of an impact inside the Dolphin half and the dependable Glynn was on hand to register a second successful penalty just shy of the half hour mark.
Slowly but surely, the hosts were starting to dominate the possession stakes and creating a number of phases close to the Dolphin line.
They could not quite break down the stern defensive resistance offered by the Munster side, but a third routine score from the kicking tee by Glynn cut the deficit to the bare minimum - 10-9 - in time for the interval.
'Belvo were starting to settle into their more accustomed rhythm and just four minutes after the restart, they hit the front for the very first time.
Further indiscipline by Dolphin once again presented Glynn with a kick in front of the posts and although 'Belvo winger Aidan Wynne found himself in the sin-bin six minutes into the half, he was joined by Dolphin lock Chris Rowe whose infringement had led to Glynn's fourth penalty of the day.
During the absence of this aforementioned duo, Glynn had his toughest kick of thegame from just inside the opposition half, but he displayed nerves of steel to move 'Belvo into the driving seat.
Either side of this brace by Glynn, Rory Scannell passed up a chance to restore the Dolphin lead, but just when it seemed like the home side were holding all the aces, Steve Ford's side once more threatened to upset the odds.
A terrific move across the 'Belvo line found its way to centre Cian Bohane and he put winger O'Shaughnessy through in the right corner. Rory Scannell missed the conversion, leaving the sides on level terms as the final quarter approached.
A massive effort was still required if 'Belvo were going to get over the finish line but thanks to powerful surges by Leinster star Leo Auva'a, Steve Crosbie and former Blues out-half Shaun Berne, they had Dolphin on the back foot for most of the closing stages.
For all their strong work in attack though, it was left to the outstanding Glynn to seal the points for Old Belvedere.
His pinpoint place-kicks in the 61st and 76th minutes, with Dolphin losing centre John Gleeson to the sin-bin, helped to set up a potential title-winning game away to Garryowen next Saturday.
Meanwhile, at Templeville Road, St. Mary's College overturned a half-time deficit with four second half tries to condemn former champions Garryowen to relegation for the first time in their history.
Garryowen had the unique distinction of being the only club to have played at the top level of the All-Ireland League every year since the competition began.
But that remarkable run was ended by a 40-26 defeat today as St. Mary's swung an enthralling 10-try see-saw of an encounter in their favour with 21 unanswered points after the interval.
Tries from JP Cooney, Mark Rowley and Steve McMahon with two Andrew O'Byrne conversions gave Garryowen a 19-12 lead in an end-to-end first half.
St. Mary's scores in the opening period came courtesy of tries from captain Kevin Sheahan and Marcus O'Driscoll plus a Cathal Marsh conversion.
But converted Dylan Fawsitt, O'Driscoll and Ryan O'Loughlin tries put the hosts firmly in the driving seat at 33-19.
Replacement Jamie Gavin converted captain Neil Cronin's try to give the visitors a try-scoring bonus point. However, they were denied a losing bonus point when St. Mary's replacement Gareth Austen crossed in the dying minutes.
Three-time league winners Garryowen had faced the prospect of relegation all season having lost their first 10 games in Division 1A.
The arrival of ex-Ireland head coach Eddie O'Sullivan sparked a mini revival with three wins, but recent defeats to Clontarf and Dolphin doomed them to Division 1B rugby for next season.
St. Mary's, on the other hand, had all but secured their Division 1A status following a bonus point victory at Lansdowne last Saturday.
Tries from Leinster pair Darren Hudson (2) and Darragh Fanning helped secure a 24-15 success for the Dubliners in the previous meeting with Garryowen at Dooradoyle back in January.
This time around, the Limerick club needed a sizeable bonus point win and for UCD to keep Ballynahinch pointless to have any chance of reaching the relegation play-off.
And the size of their task became even more daunting when number 8 Sheahan barged over on eight minutes with Leinster 'A' out-half Marsh adding the extras to put the hosts 7-0 up.
To their credit the Light Blues came storming back and tied the game in the 13th minute when prop Cooney stretched over the line and O'Byrne successfully landed his conversion.
Full-back Eamonn Connolly attempted a drop goal and flanker Anthony Kavanagh was held up as the Munster outfit continued to press for more scores.
And on 24 minutes Rowley grounded the ball off a forwards' drive to give rock bottom Garryowen a five-point lead.
Despite playing with the sun in their faces, St. Mary's hit back straight away when winger O'Driscoll crossed in the corner to level the match on the half hour.
But just four minutes later, centre McMahon slithered his way through the St. Mary's defence to cross the whitewash. O'Byrne converted to give the division's bottom side a seven-point advantage at the interval.
However, within five minutes of the restart, home hooker Fawsitt finished off an overlap and out-half Marsh landed a difficult kick to draw the teams level once again.
The hosts then regained the lead in the 54th minute when O'Driscoll touched down for his second try. Marsh converted to put the St. Mary's 26-19 in front.
They stretched the margin to 14 points on 66 minutes when centre O'Loughlin went over and Marsh brilliantly converted from the touchline.
Garryowen though refused to throw in the towel and with 10 minutes left scrum half Cronin broke free to score with replacement Gavin reducing the arrears to a converted try.
The gutsy Limerick outfit poured forward in search of a fifth try, but St. Mary's broke and were almost in when replacement Robbie Glynn over-kicked his punt out of play.
Marsh missed his penalty attempt with a minute to go, but in injury-time he made amends by converting Austen's try which sealed Garryowen's fate.
Though 47 points were scored in today's close affair at Tom Clifford Park, only four tries were split between Young Munster and 24-23 victors Cork Constitution.
Young Munster made the better start when Brian Haugh landed a penalty after only two minutes, with the visitors punished for going over the top.
The lead was short lived as Constitution broke on the left from near halfway and number 8 James Ryan went in for a try.
Midfield slackness was costly for the Cookies and another lapse after 13 minutes saw Cork Con break again from depth and this time Darragh Lyons got over in the corner. Despite the angle, Gerry Hurley converted.
In an action-packed Munster derby, the hosts' man-of-the-match Haugh touched down under the posts and added the conversion after a great break by centre Kolio Hifo.
With the last kick of the first half Hurley landed a Constitution penalty to have the visitors 15-13 in front at the interval.
Two minutes into the second half Young Munster were penalised for bringing down a scrum and Hurley landed the penalty, but it was 18-all after 58 minutes when Hifo sprinted over. Haugh converted to edge the home side back in front.
However, two more Hurley penalties gave the Bateman Cup finalists the points and Young Munster only had a losing bonus point to show for their efforts.
In the day's other Division 1A game, Ballynahinch scored three second half tries to run out 29-14 winners over UCD at Ballymacarn Park.
James Tracy and Alex Kelly both touched down for the students in the opening half, but second-from-bottom 'Hinch rallied superbly with backs Rodger McBurney and Stuart Morrow scoring a try apiece and inspirational number 8 Michael Graham finished with a brace.
However, Derek Suffern's men look destined to face a relegation play-off against Division 1B runners-up Buccaneers, as Dolphin and Young Munster are four and five points ahead of them in the table respectively entering the final round.
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