Two intelligent pieces of play from former Schools International Shane Jennings changed the complexion of this Dublin derby at Castle Avenue.
You can't put a price on the value of a reliable place-kicker, as Mark McHugh showed. Clontarf's muscular approach gave them the upper hand territorially, but the soft sod meant that it was a day for the boot.
McHugh punished Clontarf three times within 25 minutes for a 9-3 lead. All the home side had to show was a Richie Murphy penalty in the ninth minute.
The physical nature of the game was reflected in the sinbinning of St Mary's David Clare and Peter McQuillan from Clontarf.
Then, Jennings showed his hand by blocking down Murphy's punt in the middle of the field. As the ball came to earth, the St Mary's openside booted it on and, sensing the presence of Clontarf's centre Dave O'Brien, he skilfully kicked on diagonally for Gareth Gannon to jump on the ball. McHugh converted.
Just before the break, Jennings slickly pinched the ball from Clontarf centre James Downey and put Denis Hickie over beside the posts. Suddenly, it was 21-3 to the All-Ireland League holders.
Prop forward David Clare was never far from the action, and after he stormed in for the third try in the 59th minute the call came from the sideline for the fourth try that would give them the bonus point. It duly arrived 10 minutes later when O'Kelly surged over.
CLONTARF: D O'Shea; A Reddan, J Downey, D O'Brien, O Winchester; R Murphy, R O'Reilly; T Kearns, T Foucher, P McQuillan, D Sheehan, B Gissing, D Quinn, A Dignam, B Baggot. Replacements: D Rossi for Murphy, A Clarke for McQuillan, A Cullan for Dignam (all 54 mins); W O'Kelly for Foucher (64 mins).
ST MARY'S: D Hickie; B Barkley, A Conboy, G Gannon, J McWeeney; M McHugh, E McCormack; P Tucker, P Smyth, D Clare, D Griffin, M O'Kelly, T Brennan, V Costello, S Jennings. Replacements: P Lynch for McCormack (56 mins); G Hickie for Smyth, M Donnellan for Griffin (both 69 mins); J Verdon for Barkley (74 mins); B Moran for Tucker (77 mins).
Referee: D Tyndall (MRA).