Richie Lane was the hero at Castle Avenue, as his pressure penalty five minutes into stoppage time moved Clontarf up to third in the table following a thrilling encounter with Old Belvedere.
'Tarf had been hit with a sucker punch six minutes in when Belvedere full-back Danny Riordan intercepted an Evan Ryan pass inside his own '22', before running the length of the pitch to touch down in the left-corner.
Winger Josh Glynn secured a difficult conversion and was then on target with a penalty as the visitors opened up a 10-point cushion.
Clontarf were struggling to re-focus after conceding these scores, but they did give themselves hope before half-time through Tim McCoy's try, which reduced the deficit at the break to five (10-5).
Belvo's Leinster star, Leo Auva'a, almost drove over soon after the re-start, only to be forced into touch and Clontarf took advantage of this missed chance with a brace of penalties from Lane in the 59th and 68th minutes.
Belvo looked like they had done enough to seal their fourth win in-a-row when Glynn split the posts two minutes later, but 'Tarf refused to throw in the towel and, following a powerful maul deep into stoppage time, won a 30-metre penalty on the left-hand side, which Lane kicked over with some aplomb.
CLONTARF – D Fitzpatrick (N Reid 55); R Lane, N Myron, E Ryan, T McCoy; M D'Arcy, S Cronin (D Shanahan 61); B Barclay (T Furlong 51), C O'Flynn J Larbey 78), R Burke-Flynn, B Reilly, C O'Keeffe (S Crawford 63), M Garvey, A D'Arcy, F Cogan.
OLD BELVEDERE – D Riordan; J Glynn, J Kennedy, B Woods, C Murphy; S Crosbie, B O'Neill (F Lawlor 72); V Soden (G Duffy 78), M Cooney, A Howard, J Eden-Whatiri, D Moore, J Slattery, C O'Gorman (C Mallon 60), L Auva'a.
REF – O Hodges (IRFU)