Replacements: Scott Flanagan, Arthur Lyons, Steve Bradshaw, Peter Starrett, Robbie Glynn.
Replacements: Damien Attritge, John Leahy, Rob Herlihy, Chris Bannon, Eamon Mills.
A Gavin Dunne penalty on the stroke of half-time was the only score of a cagey opening half dominated by defences and a swirling rain.
Dolphin took control after the interval when centre John Gleeson burst through and out-half Rory Scannell converted to put the visitors 7-3 ahead.
But quick tries from winger Darren Hudson and hooker Dylan Fawsitt with seven further points from the boot of Dunne turned the tide in the Dubliners' favour.
St. Mary's were looking to bounce back from a 25-6 loss to Clontarf last weekend which was the Dublin club's first reversal in five matches.
Dolphin, in stark contrast, had leaked an alarming 114 points in two games and came into this fixture with the league's worst defensive record.
They did, however, come out on top in the previous meeting between the two sides back in October, when the Musgrave Park outfit won 15-10.
But with Schmidt watching on just seven days after his side's own heroics in Paris, both teams were eager to impress and started the contest at a high tempo.
Scoring opportunities were limited as each team nullified the other's attacks in the swirling rain, which forced the floodlights to be switched on.
The game's first major incident came in the 28th minute when St. Mary's prop Colm McMahon was sin-binned for infringing. Scannell, who earlier missed a drop goal attempt, kicked his penalty effort wide.
Despite being a man down, the home side matched their Munster opponents and had Steve Toal-Lennon's pass reached its intended target, they would have scored the game's first try.
The game's opening score did arrive in the final play of the half as St. Mary's, restored to 15 players, took the lead from Dunne's converted penalty on the 22.
After the restart the match came to life when Eoin O'Shaughnessy's sublime pass off a lineout sent Gleeson clean through on 54 minutes. Scannell's extras put Dolphin 7-3 ahead.
St. Mary's though clawed three points back immediately from the boot of Dunne and then turned the tie on its head with two tries in a matter of minutes.
The first of them came in the 64th minute as the home side regained the lead through Hudson's converted try in the corner.
And then just three minutes later, they increased their advantage when Fawsitt pounced on some loose back-line passing from Dolphin in defence to go over.
A fourth successful kick by Dunne put the hosts out of sight at 20-7 as the game was played out to its inevitable conclusion.
Referee: Mark Patton (IRFU)