Replacements: Will Foley, John Leahy, Ryan Murphy, Chris Bannon, Barry Keeshan.
Replacements: Rory O'Donovan, Thomas O'Reilly, Mark Donnellan, Ryan O'Loughlin, Steve Toal-Lennon.
Rory Scannell, another of Dolphin's underage international brigade, kicked them into a ninth minute lead but St. Mary's out-half Cathal Marsh was quick to cancel out that score.
The talented Marsh split the home defence to run in a superb solo try just three minutes later, with Gavin Dunne's successful conversion moving the Dubliners 7-3 in front.
Dolphin probed for a try in response and they got it midway through the first half. Killian O'Keeffe broke off the back of a scrum and linked with Cian Bohane who drew the final defender and pass for his centre partner Steve Hansel to score.
However, Scannell's successful conversion was cancelled out by St. Mary's full-back Dunne whose 36th minute penalty tied things up at 10-all for half-time.
The Corkmen exerted a territorial dominance in the second period and they went back in front thanks to a classy score from winger Darren Sweetnam in the 52nd minute.
Sweetnam broke from the 10-metre line and ran hard to beat the visitors' scramble defence, with replacement Barry Keeshan and full-back Eamonn Mills both featuring in the build-up to the game's decisive try.
The conversion was missed from wide out and Dolphin created a number of further chances that could have sealed the result.
Munster flanker David O'Callaghan charged through on a 40-metre burst that should have led to another try, St. Mary's showing impressive resilience as they remained in the hunt and looked dangerous on the counter from deep.
The Dublin outfit had an opportunity to press from a scrum, but home tighthead Paddy Ryan forced a relieving penalty for Dolphin who marched back downfield.
Keeshan missed a kickable penalty in the 76th minute but it was his terrific long range kick to touch that saw the Leesiders finish the game deep in St. Mary's territory.
They built phase after phase through their forwards but a last-gasp knock on spoiled their attempts to claim a third try right at the death.
Dolphin did not need it in the end as they made it two victories from two, sandwiching themselves between leaders Cork Constitution and third-placed Young Munster in a 'Munster 1-2-3' at the top of the table.
Referee: Stuart Gaffikin (IRFU)