A powerful first half performance that saw Cork Constitution lead 21-9 at half time and a great second half defensive proved too much for last season's league champions as the visitors ran out deserving winners.
Con' took the game to Mary's from the off and went ahead with a penalty from Johnny Holland after four minutes. Gavin Dunne had the home side level within minutes with a penalty from the touchline.
Holland restored the visitor's advantage with a second penalty on fifteen minutes.
Midway through the half winger Robert Clune got in for a try in the corner after good work by skipper Gerry Hurley and Cronan Healy to lead 11-3.
Dunne reduced the deficit on twenty five minutes with his second penalty.
Holland and Dunne exchanged penalties and two minutes before half time a great move by Con' was rewarded when Scott Deasy ran in for their second try and Holland converted for a twelve point lead at the break.
Mary's came out a different side in the second half and had Con' on the back foot for long periods. Gareth Austen got in for a Mary's try after forty eight minutes after sustained Mary's pressure close to the Con' line and Dunne converted.
Dunne was again on the scoreboard with a penalty after fifty five minutes to leave two between the sides.
Mary's came in search of a winning score but Con' withstood everything the Dublin side had to offer and eventually lifted the siege as Holland dropped an excellent goal in injury time to wrap up the victory.
St. Mary's College: Gavin Dunne; Steve Toal-Lennon, Marcus O'Driscoll, Robbie Hudson, Darragh Fanning; Philip Brophy, Matt D'Arcy; Colm McMahon, Richie Sweeney (capt), Rob Sweeney; Ciaran Ruddock, Jerry Sexton; Kevin Sheahan, Gareth Austen, Darragh Keller.
Replacements used: Conor Hogan for Lennon (25 mins), Arthur Lyons for Rob Sweeney (54), Stephen Bradshaw for Sexton (59-78 mins, temp), Dylan Fawcett for Richie Sweeney (68), Richie Sweeney for Keller (71), Mark Fallon for Austen (76), Rob Sweeney for McMahon (78).
Cork Constitution: Darragh Lyons; Robert Clune, Scott Deasy, Alex Ryan, Cronan Healy; Johnny Holland, Gerry Hurley (capt); Bryan Carney, Andrew O'Driscoll, Chris Corkery; Brian Hayes, Joe McSwiney; Graeme Lawler, James Ryan, Cathal O'Flaherty.
Replacements used: Dave O'Brien for Lawler (65-69 mins, temp sub), James Rochford for Carney (69), Niall O'Driscoll for A Ryan (71).
Referee: David Wilkinson (IRFU)