Replacements: Brian Vaughan, James Rochford, Cathal O'Flaherty, Alex Ryan, Conor Desmond.
Replacements: Tom O' Reilly, Rory O'Donovan, Mark Fallon, Andrew Burke, Rian O'Loughlin.
Hugh Hogan's side were 30-0 ahead by the 55th minute with backs Steve Toal-Lennon (2) and Matt D'Arcy sharing out the tries.
Cork Con were held scoreless in the first half. It was pedestrian enough fare until just before the interval when winger Toal-Lennon scored his brace.
Johnny Holland missed an early penalty shot for Constitution and St. Mary's centre Robbie Hudson was unable to finish off the best of few try-scoring opportunities.
Three minutes after the Hudson chance, out-half Gavin Dunne converted a 20-metre penalty to give the Dubliners a 26th minute lead.
But the complexion of the game soon changed. Referee Stuart Gaffikin sin-binned Constitution's number 8 James Ryan and St. Mary's capitalised almost immediately.
Toal-Lennon broke through for his tries after 32 and 38 minutes. The first was set up by scrum half and stand-in captain Matt D'Arcy and the second was long range intercept effort that saw him outpace the home defence.
Dunne added both conversions for a 17-0 scoreline. The Leesiders looked stunned and they were almost out for the count when Dunne landed two penalties early in the second half.
Amid an almighty battle for possession, St. Mary's flanker Ciaran Ruddock and home duo Brian O'Hara and Cronan Healy all earned yellow cards in quick succession.
D'Arcy, as busy as ever and playing a real captain's part in the absence of Richie Sweeney, then sniped over for his try as the hour mark approached.
Cork Con raised their game in the final quarter but a losing bonus point was all they could chase. A converted try from lock Brian Hayes got them off the mark in the 66th minute.
Replacement Conor Desmond's touchdown in the 79th minute was mere consolation as St. Mary's moved up two places to sixth in the table.
Cork Con have now slid to fifth overall, seven points above St. Mary's but a clear 14 adrift of table toppers Lansdowne.
Speaking to the Irish Examiner afterwards, St. Mary's coaching advisor Steve Hennessy said: "What today showed is that we have a squad capable of competing in this league. We won the AIL last season but lost a number of players at the end of it.
"It had taken us up to five years building to win the league last year and we've now started to bring in new blood.
"You saw the pace and intensity. We played some superb rugby. To score three tries on this ground and rack up 30 points against such a good side makes me very proud."
With his side having lost crucial ground in the title race, Cork Con boss Brian Walsh admitted: "It's a bit of a mountain now. It's a lesson that you must win home games, especially after having a couple of big wins on the road.
"Maybe we didn't just work hard enough, but in fairness St. Mary's played very well. They won a lot of the 50-50 challenges."
Referee: Stuart Gaffikin (IRFU)