Replacements used: Mark Fallon for Sheahan (30 mins), Arthur Lyons for McGovern (32-38, temp), Rory O'Donovan for Sweeney (32), Arthur Lyons for McMahon (40-46, temp), Sweeney for Austen (46), Lyons for Sweeney (64), Austen for Ruddock (74).
Replacements used: Gavin Ryan for Meroney (15-22 mins, temp), Michael Madden for Duggan (half-time), Greg Barry for McLoughlin (55), McLoughlin for McGarry (72).
However, it was the visitors who made the brighter start to the proceedings, as strong work from the kick-off led to a Young Munster penalty with just two minutes gone on the clock.
Scrum half Brian Haugh made no mistake with his penalty from a distance of 30 metres, and he was also on hand to double his side's advantage just six minutes later with a magnificent effort from out on the right wing.
St. Mary's were struggling to settle into the proceedings during the early exchanges, but they did receive a boost 11 minutes in when Munsters prop Ger Flaherty was sent to the sin-bin.
Gavin Dunne was wide of the mark from the penalty kick that followed, but with Marsh and winger Conor Hogan becoming more prominent in general play, the Templeogue men were forcing their opponents onto the back foot for the first time in the contest.
The Limerick outfit managed to survive during Cotter's absence from the play, but their defence was finally breached on 24 minutes when Marsh finished off a lengthy period of possession in the 22 with a try in the left corner.
Full-back Dunne was narrowly off target from an awkward conversion, but it looked like St. Mary's had gained the impetus to push on for the remainder of the first half.
Young Munster responded to the concession of this score in devastating fashion, though, as second row Alan Kennedy charged down Chris Lilly's attempted clearance moments later which enabled Derry O'Connor to drive over on the left hand side.
This was a hammer blow for the hosts, and with the deadly accurate Haugh adding to his superb conversion with a routine 34th minute penalty, Young Munster held a hard-earned 16-5 cushion at the interval.
A strong showing upon the resumption was needed if St. Mary's were going to force their way back into the reckoning.
Although the usually reliable Dunne failed to open his account with a close-in place-kick three minutes after the restart, good approach play by Mark Sexton and Brian McGovern led to a much-needed Ryan O'Loughlin try from the next attack.
Dunne was unable to add the extras from the right wing, but he eventually got it right 12 minutes into the second period with a penalty from just outside the 22-metre line.
An outstanding kick from distance by Haugh then breathed fresh life into the away side's challenge, but their lead was reduced to the bare minimum just a minute later when Colm McMahon burst through for an unconverted try.
Unfortunately for the influential McMahon, his afternoon came to an end 17 minutes from time when he was given his marching orders by referee Peter Fitzgibbon after picking up his second yellow card of the day.
This was a huge setback for St. Mary's heading into the closing stages and despite the Dubliners' best endeavours, a stoppage time try from Hugh McGrath - expertly converted by the terrific Haugh - gave Young Munster a very encouraging opening day victory.
Referee: Peter Fitzgibbon (IRFU)