Eddie Devitt’s injury-time try saw Old Belvedere deny Clontarf right at the death in Saturday afternoon’s Ulster Bank League Division 1 semi-final at Anglesea Road.
Clontarf outscored the hosts by three tries to two and a 79th minute penalty from Daragh O'Shea looked to have won it for the north Dubliners.
But 'Belvo conjured up an opportunistic match-winning try, with player-coach Andy Dunne scooping a long pass out to winger Eddie Devitt who touched down after a clever chip and chase.
Dunne's missed conversion was the final kick of a pulsating Dublin derby, during which Clontarf had the game's outstanding player in RTE/Ulster Bank man-of-the-match Barry O'Mahony.
Tries in the first quarter from Killian Lett (5 minutes) and O'Mahony (15) gave Clontarf a deserved 12-0 advantage as Old Belvedere struggled early on.
Nonetheless, Phil Werahiko's men soon got to grips with 'Tarf's beefier pack and three penalties from Conal Keane, the last coming into the 37th minute, closed the gap to three for half-time.
This semi-final encounter witnessed an early bout of kicking, with full-backs Daniel Riordan and Daragh O'Shea striking the ball well. Old Belvedere had a slight wind behind them.
Colm O'Shea probed on the right before play was switched out to the left through a long, search pass from Martin Dufficy.
Dufficy and Evan Ryan put Lett into space and the centre used his strength and pace to get past Devitt and Keane and crash over in the corner. Dufficy's conversion attempt was off target.
Aaron Dundon, who was recently capped by Leinster, combined well with one of the province's former locks, Ben Gissing, and youngster Michael Kearney to offer Clontarf a solid platform in the lineout.
A late challenge by Ben Marshall on Dufficy as the Clontarf out-half kicked through gave the visitors a penalty which Daragh O'Shea sent to the left of the posts from 40 metres out.
Bernard Jackman's side were almost immediately back on the attack. O'Mahony gathered an overthrown lineout near halfway and popped a short pass for Lett to raid through the middle.
The Division 1B table toppers recycled possession and drove up to five metres from the try-line before a deft pass from scrum half Hugo Nolan put flanker O'Mahony over from close range. Dufficy added a terrific conversion from the left.
Old Belvedere regained their composure to work the ball through their fowards, driving at the heart of the 'Tarf defence and into the 22.
After good work by captain Richie Leyden and Jason Risdon, Clontarf prop Ciaran Kavanagh was pinged for not rolling away after the tackle in the 21st minute and Keane landed the resulting kick from close in.
Eight minutes later, Keane added a second penalty with a well-struck kick from further out on the right. 'Belvo tried to get David Mongan involved on the left wing, but there was very little room for him to manoeuvre in.
Indeed, winger Mongan then fell awkwardly in a double tackle from Colm O'Shea and Eoin Cremen, prompting a reshuffle of the hosts' back-line where Chris Keane came in at scrum half and Simon Keogh moved out wide.
A poor kick out of his 22 from Lett gave possession back to Old Belvedere and although Jonathan Slattery was hauled down short of the line after an initial surge from Stewart Maguire, Keane tagged on his third successful kick.
12-9 ahead at the interval, 'Tarf made a superb start to the second half when the strong-running O'Mahony darted through for his second try after just 87 seconds.
The visitors moved the ball at pace, Cremen was stopped short on a powerful break. But Dufficy then stretched the 'Belvo defence with a long pass out to the left where O’Mahony sped through a gap between the covering Devitt and Marshall to score.
Dufficy's missed conversion was followed by another penalty from Keane which left five points between the sides with just over half-an-hour remaining.
Suddenly, there was huge pressure on 'Tarf when their winger Michael Keating was yellow carded for tackling Riordan in the air – his second such offence.
Riordan blasted through a hole in the 'Tarf defence and Keogh just had a foot in touch as 'Belvo tried to break their try deadlock.
The wily Dunne was introduced at out-half for the hosts and he lost possession in a strong tackle from Evan Ryan as 'Tarf desperately defended their line.
'Belvo also lost a number of lineouts in promising positions, one of them coming after Clontarf replacement lock Ben Reilly was sin-binned for illegally pulling down a maul just moments after his introduction.
Down to 13 players, Jackman's charges were really up against it. However, they showed great resilience and Simon Crawford's tackle on Devitt, which prevented a certain try, summed up their spirit.
They gave way eventually on the hour mark. A terrific cut-out pass from Dunne put number 8 Leo Auva'a rounding in from the right for a much-needed try.
Auva'a had managed to get in closer to the posts, but Dunne pulled his conversion attempt wide of the target and the scoreboard stood at 17-17.
The action was frantic, holding the interest of a vocal crowd which included Clontarf native and Ireland captain Brian O'Driscoll.
Restored to 15 players, the momentum seemed to swing back in 'Tarf's favour. They got their maul going again and Justin Wilson, who came on for the injured Dufficy, jinked his way into the 22.
Daragh O'Shea pushed a hurried drop goal wide of the posts and then had a similar effort in the 76th minute charged down by Auva'a.
But the experienced full-back came up trumps three minutes later, punishing John Kennedy for a ruck offence just outside the 'Belvo 22.
The win was within Clontarf's grasp, and they duly won a penalty just inside their half with time running out. Crucially, O'Shea failed to find touch with his kick.
Auva'a gained ground on a sidestepping run out of his 22 and with play brought up to halfway, Dunne fired out a pinpoint pass, inviting Devitt to chip over the top with only Crawford covering the danger.
The ball dropped, five metres out, and was controlled by Riordan's outstretched left boot, teeing it up for Devitt to gather and touch down.
So, delight for this dogged Old Belvedere side. They are through to their first Division 1 final, at Donnybrook on Sunday, May 1, after putting last year's semi-final disappointment behind them.
But spare a thought for promoted Clontarf who had to endure a heartbreaking finish to their campaign.
Referee: John Carvill (IRFU)