While many felt this game should have gone to a replay, another meeting between these two great sides would certainly have doubled the attendance that watched this encounter.
The sides were level at 19-19 after 80 minutes and while two halves of extra-time failed to produce a score, but this was a great advert for club rugby.
Shannon got off to a great start with Tadhg Benett kicking them into the lead with a drop goal after fifty three seconds.
Clontarf's skipper Darragh O'Shea was off target with a long range penalty attempt but had the sides level on fifteen minutes with his second attempt.
Both Bennett and O'Shea exchanged penalties to leave the sides level 6-6 at the end of the first quarter and than came the game's opening try that was to prove crucial on twenty three minutes
A break by Bennett set up David O'Donovan who touched down in the corner. Clontarf hit back immediately with Heinrich Stride touching down after some excellent work by O'Shea and Martin Dufficy but O'Shea missed the conversion.
Shannon hit the front again when O'Donovan touched down after the Limerick side took a quick tap penalty eight minutes before the break for a 16-11 interval lead.
The sin binning of Shannon's Padraic O'Brien early in the second half proved crucial for the Dublin side as from the resulting penalty O'Shea reduced the deficit and three minutes later the Leinster side roared on by a sizeable support hit the front with a try from Simon Crawford after great work by scrum half Paul O'Donohoe. O'Shea missed the conversion.
The Dublin side now looked like champions but this saw Shannon raise their game and Bennett tied the scores with a penalty as a result of Clontarf's Niall O'Brien been shown a yellow card for coming in at the wrong side of a ruck.
Extra time produced a pulsating twenty minutes with the nearest to a score came from Clontarf's substitute Max Rant-McDonald who lost possession as he went to ground the ball with six minutes remaining.
Scorers: Clontarf: Heinrich Stride, Simon Crawford tries, Daragh O'Shea 3 pens.
Shannon: David O'Donovan 2 tries, Tadhg Bennett 2 pens, 1 Drop Goal.
CLONTARF: Phil Howard; Niall O'Brien, Daragh O'Shea (capt), Breffni O'Donnell, Michael Keating; Martin Dufficy, Paul O'Donohoe; Kenny Dorian, Aaron Dundon, Niall Treston; Ben Reilly, Simon Crawford; Heinrich Stride, Niall Carson, Martin Garvey.
Replacements used: Johnny Wickham for Dorian (61 mins), Max Rantz-McDonald for O'Brien (72), Conor Keegan for O'Donnell (80), Ben Focas for Carson (90).
SHANNON: David O'Donovan; Richie Mullane, Fionn McLoughlin, John Clogan, Stephen Kelly; Tadhg Bennett, Frankie McNamara; Killian O'Neill, Mike Essex, Kevin Griffin; Padraic O'Brien, Fergal Walsh; Donnacha Ryan, Eoghan Grace, David Quinlan (capt).
Replacements used: Les Hogan for O'Neill (60 mins), Emmet McLoughlin for O'Brien, Marcus O'Driscoll for Kelly (both 90).
Referee: Alain Rolland (IRFU)