Leinster lost control of their URC destiny as Ulster struck late to claim a much-needed victory that confirms a play-off place.
Needing a win to return to the top of the standings, Leo Cullen's second-string outfit almost held out for the victory but were ultimately undone by their own mistakes and failure to capitalise on Ulster’s.
Richie Murphy, confirmed as permanent head coach during the week, will be delighted with how his charges stayed in the game and claimed four priceless points that sees them leapfrog Edinburgh into sixth with one round remaining.
With Edinburgh and Benetton - seventh and eighth - meeting next week, only of them and Lions, should they claim five points against the Bulls, could overtake Ulster but the lowest they can finish is eighth.
Leinster, meanwhile, are in third, and will be unable to improve on that position if Munster and Bulls win their round-18 games.
Ulster’s players did a lap of honour as they thanked the home support in their last regular season game at the Kingspan Stadium with John Cooney hailed as the hero with his last-gasp kick the winning of the game and completing a double over Leinster.
The good news for Leinster ahead of next weekend’s Investec Champions Cup final against Toulouse in London, was the return of both Hugo Keenan (below) and James Ryan, who both lasted the full 80 minutes, neither showing any rustiness after hip and arm injuries, respectively.
However, Charles Ngatai and Tommy O'Brien were injured in the first-half and Leinster will be sweating on their fitness ahead of the game next weekend.
The hosts dominated the proceedings for the majority of the opening half but still, somehow, found themselves trailing 14-10 going in.
Leinster's lead came despite a shaky lineout, a dodgy scrum and a series of unforced errors including a restart that went out on the full, poor handling, and indiscipline at the breakdown.
It was the returning Keenan who managed to keep the score respectable in the early exchanges, firstly denying the lively Mike Lowry with a textbook tackle after which the wing regathered but not before getting to his feet.
Later, the Ireland full-back was on hand to soak up Will Addison who butchered an overlap close to the line.
In between those saves, Ulster did open the scoring when, after Lowry was stopped again, Herring directed a lineout maul over the line and this time the TMO couldn’t save Leinster.
The visitors couldn’t find a rhythm but suddenly found themselves level, their score coming moments after Rónan Kelleher overthrew a lineout.
Billy Burns was blocked down in midfield by Will Connors and the flanker had Ngatai (above) in support. The centre brushed off a weak tackle attempt by the chasing out-half to score under the posts.
Back rows Cormac Izuchukwu and Nick Timoney were running hard at the Leinster line and gaining ground with each carry, while Leinster lost Ngatai and Tommy O’Brien to first half-injuries.
Then Leinster grabbed their second try off a ruck that Ulster appeared to have well marshalled. Byrne passed inside to Keenan coming at a perfect angle and he evaded the Ulster defence before instinctively feeding the ball back inside where Cormac Foley was the support runner. Byrne added his second conversion to put Leinster seven ahead.
But Leinster couldn’t manage the restart and after a couple of Ulster phases, went off their feet at the breakdown allowing Cooney to tap over a penalty.
The scrum-half repeated the trick four minutes into the second half after Ryan was penalised at the breakdown and there was just a single point between the sides.
But Leinster responded with a third try moments later, eventually wearing down Ulster’s ruck defence with a series of carries that ended with Ala’alatoa forcing his way over beside the posts to make Byrne’s conversion a formality.
Ulster looked tired and Leinster went through the phases looking to land a killer blow but Byrne lobbed out a reckless pass that landed in Jacob Stockdale’s hands and the Ireland winger ran from his own 22 to score with Foley, deployed on the wing, making a forlorn effort to chase him down.
Cooney converted to make it a one-score game with 20 to play and Leinster conceded a scrum penalty soon after to a huge cheer from the majority of the sold-out stadium.
Byrne almost made amends for throwing the intercept by picking off a loose pass but the out-half fumbled to let Ulster off.
The mistakes kept coming, with neither side innocent of sloppy errors and flat-footed demeanour.
Thoman Clarkson, Ross Molony and Scott Penny arrived to add some energy but Ulster delivered a superb defensive set to turn the ball over close to their own line.
Cooney targeted Rob Russell with a set of box-kicks as Ulster worked their way down field but a forward pass from Lowry to Stuart McCloskey halted their progress.
But they refused to give in and forced a penalty just inside their own half when Leinster defenders were too slow to roll away.
With 80 seconds on the clock, Cooney’s effort sailed over the posts and that was that.
Ulster: Ethan McIlroy; Mike Lowry, Will Addison, Stuart McCloskey, Jacob Stockdale; Billy Burns, John Cooney; Eric O'Sullivan, Rob Herring, Tom O'Toole; Kieran Treadwell, Alan O'Connor (capt); Cormac Izuchukwu, David McCann, Nick Timoney.
Replacements: Tom Stewart, Andy Warwick, Scott Wilson, Harry Sheridan, Matty Rea, Nathan Doak, Stewart Moore, Jude Postlethwaite.
Leinster: Hugo Keenan; Tommy O'Brien, Jimmy O’Brien, Charlie Ngatai, Rob Russell; Harry Byrne, Cormac Foley, Cian Healy, Rónan Kelleher, Michael Ala’alatoa; James Ryan (capt) Brian Deeny; Max Deegan, Will Connors, Jack Conan.
Replacements: John McKee, Michael Milne, Thomas Clarkson, Ross Molony, Scott Penny, Luke McGrath, Sam Prendergast, Ben Brownlee.
Referee: Andrea Piardi (FIR)
