Replacements: Jonathan Larbey, Tadhg Furlong, Simon Crawford, Sam Cronin, Matt D'Arcy.
Replacements: Arthur Lyons, Rory O'Donovan, Mark Fallon, Conor Hogan, Andrew Burke.
With Shannon beating UL Bohemians, St. Mary's are now four points clear of the Bohs below them and level with Old Belvedere who play tomorrow.
St. Mary's had the better of the opening exchanges but Gavin Dunne's early penalty sailed wide of the mark in what were difficult conditions for the kickers. He made up for it after ten minutes when Clontarf were again penalised.
Clontarf, who have had wingers Michael McGrath and Tim McCoy in fine try-scoring form, came back and were unlucky to be held up short as their pack drove towards the visitors' line.
When St. Mary's were penalised for offside it fell to Leinster's Noel Reid to cancel out Dunne's earlier effort, making it 3-3 after 17 minutes.
From the restart Cathal O'Flynn knocked on and it put Clontarf right back on the defensive. The ensuing passage of play ended with another successful Dunne penalty.
Then came a moment of magic from Reid. He jinked his way past three would-be tacklers before offloading to McGrath who touched down for his sixth try of the season. Reid failed with his conversion kick from wide on the right.
St. Mary's replied with yet another penalty from Dunne who capitalised on Killian Lett's illegal tackle on Marcus O'Driscoll to put Hugh Hogan's men back in the lead.
With Lett in the sin-bin, St. Mary's upped the ante before half-time and prop Rob Sweeney barged over for a timely. Dunne's conversion had St. Mary's 16-8 up at the break.
With centre Lett back on the pitch, Clontarf had the better of the possession stakes in the second half and another Reid penalty, seven minutes in, put them right back into the game.
But although they continued to dominate, 'Tarf failed to turn that domination into points on the scoreboard. The second opportunity of the half fell to Reid, but his effort sailed wide after the hour mark.
He made no mistake five minutes later when Robbie Hudson was penalised for a high tackle, closing the gap to 16-14. Dunne managed to responed with a well-taken penalty on 73 minutes.
But Clontarf were not done. With just minutes remaining, St. Mary's conceded a penalty and Reid opted to have a shot at goal and he brought 'Tarf back to within two points again - 19-17.
It made for a grandstand finish in which Clontarf twice worked the ball deep inside the St. Mary's 22 but when it came back to Fitzpatrick, his drop goal attempts - two in as many minutes - both sailed wide, the latter from just under the posts.
Referee: Mark Patton (IRFU)