ST. MARY'S COLLEGE 19 DOLPHIN 11, Templeville Road
Scorers: St. Mary's College: Tries: Mark Sexton, Frank Lynch, Matt D'Arcy; Cons: Mike Skelton, Frank Lynch
Dolphin: Try: Andrew O'Driscoll; Pens: Barry Keeshan 2
The result means Dolphin remain three points behind fourth-placed Clontarf in the Division One standings, with one round of matches remaining.
The loss for David O'Mahony's side was their first in six league games and means they will have to beat table toppers Cork Constitution on April 19 to stand any chance of making the play-offs.
But a win for 'Tarf at home to Galwegians on the same date would put the Dubliners through, no matter how Dolphin fared at Temple Hill.
Dolphin, who included their Munster-contracted back rower James Coughlan, made all the early running and they put a lot of pressure on a Mary's side that had nothing but pride to play for.
The visitors opened the scoring in the 15th-minute when Mary's were punished for tackling a player without the ball and out-half Barry Keeshan stepped up to land his first penalty.
Mary's needed Philip Brophy to pull off a try-saving tackle to prevent them from going further behind.
But the home side then lost back rower Kevin Sheahan to the sin-bin, in the 25th-minute, and Keeshan landed the resulting penalty kick.
Dolphin led 6-0 at the break and were good value for that lead, although they should really have been further in front.
Mary's missed a 44th-minute chance to open their scoring account but number 10 Mike Skelton was wide with a penalty.
A couple of minutes later, Dolphin were over for their only try thanks to a training ground move.
They kicked a penalty to the corner, hooker Andrew O'Driscoll worked a quick lineout, getting the ball back after a quick one-two, and he had the pace and power to pile over the line from ten metres out.
Keeshan missed the conversion from wide out on the left and Mary's, suddenly playing with with greater freedom, got themselves into the game.
The pacy Daragh Fanning was stopped just short of Dolphin try line before the momentum was with Mary's and off a subsequent ruck, Mark Sexton, brother of Leinster out-half Jonathan Sexton, went over under the posts.
Skelton converted for 11-7 and after Dolphin tighthead prop Christy Condon was sin-binned, Mary's kicked to the corner before stretching the visitors' defence and putting replacement out-half Frank Lynch in to touch down close to the posts.
Lynch converted his try to give the Dubliners a three-point buffer and Dolphin, try as they might, just could not reply.
With time running out, some poor handling and bad decision-making let the visitors down and credit much also go to the Mary's defence which was outstanding.
The hosts, who ironically face 'Tarf in the Leinster Senior Cup final next Saturday, sealed the win and robbed Dolphin of a losing bonus point with a third try.
Leinster prop Juan Gomez barged his way forward, set up a ruck in the visitors' 22 and scrum-half Matt D'Arcy sped over the whitewash in the next phase.