Replacements used: Martin Garvey for Ryan (33-40 mins, blood sub), Peter du Toit for Cronin (53), Garvey for Ryan (61), Timmy McCoy for O'Shea (61-73), McCoy for Ryan (82).
Replacements used: Michael Cromie for Taylor (44 mins), Jamie Kirk for Simpson (61).
Old Belvedere were overwhelming favourites to secure their second league crown in four seasons, but their surprise 23-18 reversal in a televised encounter with Garryowen made it possible for Clontarf to reach the promised land.
And Andy Wood's men displayed great mental strength to eventually overcome the considerable challenge of Ballynahinch in a 27-13 victory at Castle Avenue.
'Tarf had lost their last two games to UCD and Lansdowne, but back at their north Dublin base they returned to winning ways on one of the most dramatic afternoons in the league's history.
Having overcome Belvedere in a recent top of the table clash at this venue, 'Tarf could have afforded to lose one of these games against UCD and Lansdowne.
However, narrow losses to the south Dublin pair left 'Belvo in the driving seat ahead of this final weekend of the season. If Clontarf's form was worrying heading into round 18, the polar opposite applied to Ballynahinch.
Their impressive winning run - with three bonus point triumphs on the trot - ensured that automatic relegation was ruled out of the equation, while they came south with an outside chance of avoiding a promotion/relegation play-off with Buccaneers.
A sixth league win for 'Hinch would see them leapfrogging Dolphin - if the Corkmen lost by more than seven points at home to Lansdowne - with seventh-placed Young Munster also in their sights.
Indeed, they started this game in fine spirits and after dealing effectively with a number of kicks from deep by Clontarf, 'Hinch opened the scoring thanks to a routine penalty by centre Richie Lobb.
But the home side hit back with a 10th minute try as outstanding build-up work from back row pair Adrian Darcy and Karl Moran paved the way for winger Michael McGrath to power over the line.
With dependable out-half David Joyce knocking over the conversion, 'Tarf had the early advantage that they craved.
After the edgy early moments, Clontarf were now making significant headway inside the opposition half. Joyce was unfortunate to see a 17th minute penalty rebounding off the post, but he made no mistake from a similar position moments later.
'Hinch constantly found themselves on the retreat as the first half wore on, but they managed to hold firm during a tricky period. Lobb's second successful kick at goal on the half hour brought their deficit back to four points.
Indeed, Derek Suffern's side looked set to make it a one-point game seven minutes before the interval, but facing into a tricky breeze, Lobb missed the target on this occasion.
This was something of a let-off for the north Dubliners, and Joyce was able to take advantage when converting a close range penalty to give his side a 13-6 interval buffer.
With news filtering through that 'Belvo were trailing by the same margin, the Clontarf faithful had reason to be optimistic.
A dogged Ballynahinch threw a spanner into the works though, as scrum half Aaron Cairns dived over on the right for a try four minutes into the second half.
An excellent touchline conversion by Lobb restored parity and added another dollop of tension to proceedings.
The game remained delicately poised moving into the final quarter and although 'Tarf were enjoying the majority of possession, a number of basic errors released the pressure on their County Down opponents.
The hosts' perseverance was eventually rewarded and it was replacement scrum half Peter du Toit who proved to be the hero of the hour.
Just twelve minutes from time, the former Ireland Under-20 international touched down at the end of a telling move on the left for 'Tarf's second converted try of the day.
This effort lifted a huge weight off the Clontarf shoulders and despite holding firm amidst intense pressure, 'Hinch's defence gave away again in the 80th minute as du Toit broke throught the cover to complete his brace.
There was an anxious wait to see how the match at Dooradoyle panned out but after Garryowen held off a late 'Belvo charge, Clontarf were able to bask in the glory of their historic league success.
Referee: George Clancy (IRFU)