Replacements used: Colm McMahon for Lyons (half-time), Stephen Bradshaw for Fallon, Ian O'Neill for Dunne (both 53 mins), Dunne for O'Neill (63), Tom O'Reilly for McGovern (69), Robbie Glynn for D'Arcy (72), D'Arcy for O'Driscoll (80).
Replacements used: Bryan Byrne for Culleton, Ian Hirst for Burke-Flynn (both half-time), Tom Byrne for Murphy, Peter du Toit for Cronin (both 52 mins), Burke-Flynn for Furlong (60).
The Templeogue outfit eventually prevailed under the stewardship of Peter Smyth, who is back at the helm following Hugh Hogan's one-year stint as head coach, but while St. Mary's have struggled for consistency since their domestic success, Clontarf have continued to challenge towards the summit of the Division 1A table.
Indeed, they began the weekend just three points behind league leaders Old Belvedere with a game less played, and were hoping to maintain their title push with their 11th victory from 13 outings.
However, the form of the hosts has improved significantly in recent weeks, and they forced their north Dublin opponents onto the back foot in the early moments.
Forceful play by the St. Mary's attack led to a fourth minute penalty from a distance of 40 metres, and with a strong wind at his back, reliable full-back Gavin Dunne split the posts off his trusty right boot.
This was the kind of start to the proceedings that was badly needed from the home side, and with 'Tarf finding it difficult to settle into their familiar stride, St. Mary's sensed there was an opportunity to further enhance their scoring tally.
Out-half Cathal Marsh, a rising star for Leinster at provincial level, almost broke through for an opening try 16 minutes in, but owing to an infringement by home lock Conor O'Keeffe, referee Michael Black awarded a routine penalty to St. Mary's in front of the posts.
As expected, Dunne found the target which meant that Clontarf found themselves six points down at the end of the first quarter.
However, tt was only a matter of time before Andy Wood's charges had their own period of dominance.
Although St. Mary's initially soaked up the pressure applied by the visiting team, a sweet cross-field move just shy of the half hour mark was finished with style and aplomb on the left wing by the speedy Max McFarland.
This score certainly helped to settle the 'Tarf nerves and although David Joyce was well short of the mark from a tricky conversion kick into the breeze, they had reduced the St. Mary's lead significantly
In a stop-start conclusion to the opening period, Dunne opted to try his luck from a penalty inside his own half but despite his best efforts, he could not carry his kick between the posts.
As a result, St. Mary's brought a slender 6-5 advantage into the second half. But with Clontarf opting to introduce Bryan Byrne and Ian Hirst upon the resumption, a massive effort was needed from the southsiders.
Ireland Club international Joyce did pass up an excellent chance to give 'Tarf the lead for the very first time from a right-angled penalty attempt five minutes after the restart, but St. Mary's continued to probe with plenty of intent.
Yet, Clontarf were seeing more of the ball in the third quarter than they had for much of the first half and inevitably, a successful 35-metre Joyce kick after 52 minutes finally edged the favourites into the ascendancy.
As the half wore on, both sides opted to introduce further fresh legs into the fray, but just when it looked like a tightly-contested finale was on the cards, lethal right winger Michael McGrath pounced for Clontarf's second try 15 minutes from the end.
This proved to be a morale-sapping moment for St. Mary's and after Joyce added his second penalty nine minutes from time, replacement hooker Bryan Byrne finished off an enterprising passage inside the St. Mary's 22 to provide Clontarf with yet another precious win on the road.
Referee: Michael Black (IRFU)