Replacements: Cian Culleton, Tom Byrne, Keith Donoghue, Peter du Toit, Killian Lett.
Replacements: Leo Auva'a, Adam Howard, Brian Coyle, Fergal Lawlor, Shaun Berne.
In truth it was a deserved success for Clontarf, who go top of the league on better scoring difference and still have a game in hand over second-placed Old Belvedere.
Despite dominating territory, Clontarf trailed 10-8 at the interval after two breakaway tries from 'Belvo winger Philip O'Dwyer and full-back Josh Glynn.
A Darragh Fitzpatrick try in between those scores and a David Joyce penalty on the stroke of half-time made it a two-point game.
Following O'Dwyer's sin-binning, hooker Bryan Byrne grounded the ball from a maul off a lineout and David Joyce kicked the extras to put 'Tarf 15-10 up.
Old Belvedere reduced the deficit with a Steve Crosbie penalty, but the home side crossed again through a forwards' drive with Ian Hirst getting the ball down.
The visitors applied late pressure, but Clontarf defended resolutely to secure the victory which now makes them firm favourites to be crowned league champions next month.
When the two clubs last met back in September on the opening day of the season, Paul Cunningham's Belvedere side fought back from 15-0 down to win 24-21 at Anglesea Road.
Little did either team know back then that their next meeting would be a potential Division 1A title-deciding clash - dubbed 'the big one' in some circles.
Just three points separated first and second before kick-off as both sides came into this fixture in rich veins of form.
Clontarf had won 12 of their 13 league matches since that curtain-raising loss, while 'Belvo were unbeaten in 12 games between early October and last week.
Something had to give and both set of players, fully aware of what was at stake, went at each other right from the first whistle.
The Andy Wood-coached hosts applied the early pressure with most of the game being played inside the 'Belvo half.
Michael McGrath almost gave the hosts a deserved lead on 14 minutes after breaking through a series of tackles and chipping the ball forward, but the bounce of the ball cruelly alluded the Ireland Club international winger.
However, against the run of play, the visitors broke quickly to score just two minutes later. A sweeping back-line move out wide, in broken play, sent O'Dwyer on his way as he collected his own punt forward to go over for the opening try.
Joyce had a chance to reduce the deficit on 23 minutes, but kicked his penalty attempt just wide as the hosts kept the pressure on.
But the north Dubliners' dominance was finally rewarded on the half hour when prop Royce Burke Flynn powered through a challenge before passing for Fitzpatrick to draw the teams level.
Again 'Belvo broke immediately to make it 10-5 when the ball was played out to the left and fed inside by Jack Conan for Glynn, who started in place of injured captain Daniel Riordan, to cross in the far corner.
The home side fought back and were held up over the line in the corner before it was a case of third time luck for out-half Joyce with his only successful penalty of the night.
Clontarf continued to dominate possession after the break, but were being met by stubborn resilience from the 'Belvo defence.
Then came the turning point of the match when approaching the hour mark, Belvedere winger O'Dwyer was sin-binned for deliberately knocking the ball on as Clontarf looked odds-on to score.
Wood's charges took full advantage of the extra man from a forward maul off the lineout which resulted in in-form front rower Byrne crossing the whitewash. Joyce converted to put the hosts 15-10 ahead.
Although a Crosbie penalty in the 64th minute reduced the arrears, 'Tarf extended their advantage five minutes later.
Loosehead prop Hirst crashed over following a powerful drive from the pack to put the hosts 20-13 ahead as the rain came pouring down.
Much like the rain, Old Belvedere flooded forward in desperate search of a converted try.
However, 'Tarf battled right to the end to seal a hard-fought home win which was watched by a big crowd - including local favourite and Six Nations winner Cian Healy.
Referee: Leo Colgan (IRFU)