Replacements: Bryan Byrne, Ben Barclay, Adrian D'Arcy, Ronan Dillon, Niall Myron.
Replacements: Rory O'Donovan, Arthur Lyons, Mark Donnellan, Conor Hogan, Steve Toal-Lennon.
St. Mary's are going through a transitional period and are shorn of a few of the old guard due to injury and retirement. It did not help their cause that two of their key players, new out-half Cathal Marsh and captain Darragh Fanning, were also absent on Leinster duty.
Their north Dublin hosts too were short a few selection options but are settling as a squad. With home advantage and the usual vocal support, 'Tarf were admittedly in a better position to overcome any shortages.
The first half was played at a frantic pace with both sides struggling to control the slippery ball. Half-time saw the sides locked level at 6-all after two penalties apiece from David Joyce and Gavin Dunne.
St. Mary's did benefit from the inclusion of Leinster's Darren Hudson at full-back, although his attacking opportunities were limited.
This tight contest was a battle between the forwards for the most part, with early scrum penalties seeing Joyce (3 minutes) and Dunne (18) both split the posts.
The visitors got their lineout maul going on the half hour mark, inspired by a fine take by flanker Barry O'Flanagan, and a subsequent infringement by 'Tarf allowed Dunne to boot his side in front. Four minutes before the break, Joyce equalised for 'Tarf.
The second half came to life when Clontarf winger Michael McGrath was fired into the St. Mary's secondary defencee only to be hauled down by a superb tackle with the try-line in sight.
The Clontarf pack then took over and multiple phases of driving finally saw replacement hooker Bryan Byrne grind his way over beside the posts with Joyce converting for a 13-6 lead.
A classic home score followed. Off a creaky scrum on the right number 8 Tony Ryan took the ball down the blindside at greace pace, popped it to scrum half Sam Cronin and crucially stayed with him thereby creating enough confusion in the defence for Cronin to give the inside dummy and outside flip for McGrath to make the corner.
Unfortunately for the home fans, Andy Wood's men then got wobbly with the restart and lost possession as they were moving upfield.
In true St. Mary's style, the visitors made hay on the turnover and Steve Toal-Lennon scored a try right out of the 'Barry John' scrapbook to keep the game alive and the crowd quiet.
Toal-Lennon attacked from inside his own half, stepping and speeding into space before beating brilliantly two defenders and outsprinting the cover to run in a very classy solo effort.
That converted score closed the gap to 18-13, but 'Tarf managed to hold on through some scary moments around the fringes with number 10 Joyce kicking a clinching brace of penalties.
The game ended with a breakout into the St. Mary's 22, an infringement at a maul, some handbags and eventually two yellow cards (awarded to Simon Crawford and Matthew D'Arcy). A penalty was awarded to the home side which Joyce converted to complete his 14-point haul.
Referee: Nigel Correll (IRFU)