Ulster had started off the brighter side and took the lead, only for Leo Cullen's side to grab two tries to lead at half-time. Leinster looked to have landed a killer blow early in the second half with their third try, but an inspired try from Jacob Stockdale and a late John Cooney penalty from distance secured a famous victory for Richie Murphy's team.
With the match sold out days in advance, it was a huge occasion, with Ulster's season on the line on the last home game of the regular season. The fans were right up for the derby, flooding in early as the sunshine kissed Belfast on a glorious night for some top Irish rugby.
Before kick-off a moment's silence was held to remember those who have passed away in the rugby community, which was observed impeccably.
Ulster started on the front foot, being driven on by the home support. Mike Lowry started off like a man possessed, with Ulster's latest centurion having two try attempts chalked off.
The first for narrowly being second to the ball with the Leinster winger, Russell, and the second chalked off for a double movement after a contest with Hugo Keenan who defended well.
The home side made their pressure count after 13 minutes, with a well worked lineout and an excellent driving maul that saw Rob Herring get the ball down. The try was given after a TMO check. Cooney converted for an early 7-0 lead.
Leinster then showed their ruthlessness and why they are a top side, with two quickfire tries to leave Ulster supporters stunned.
The first was a charge down from Will Conners on a Billy Burns kick that saw Charlie Ngatai race clear to finish. The second, was a skilful piece of play from Hugo Keenan, who spun and passed to Cormac Foley, who ran through to score their second. Harry Byrne converted both tries.
Ulster's mistakes were punished by a ruthless away team. Ulster kept pushing and opted to take the three points on offer before half-time with a penalty under the posts. John Cooney made it a four point game at 10-14 going into the break.
HT: Ulster 10-14 Leinster
After the restart Ulster started off well, and quickly snatched another penalty to claw back the score. John Cooney's strike made it a one point game with plenty of the game to go.
Leinster grew into the half, and showed signs of their attacking threat. Their pressure resulted in a third try, after great work from their forwards, who drove towards the line after a scrum penalty. Tighthead prop, Michael Ala'alatoa, was the ball to get it over the line for Leinster and the converted try felt like a killer blow to Ulster.
Then, a moment of magic. With Leinster chasing a fourth, bonus-point try and going through the phases, a lob out wide from Harry Byrne was intercpeted by Jacob Stockdale who read the flight of the ball perfectly. The big winger pinned his ears back and ran like Ulster's season rested on his shoulders. He had the engine and pace to sprint clear for a fantastic score. Cooney's conversion was on the money and Ulster felt a sweep of momentum.
Ulster showed plenty of grit, as Leinster looked close to another try, only for some heroic defence. First from Mike Lowry, who hit back-rower, Max Deegan, into touch and prevented an almost certain try.
Then a huge moment as Leinster pushed towards the tryline, ulster threw their bodies on the line and replacement hooker, Tom Stewart, made a vital jackal turnover.
With Ulster feeling the comeback on and belief through the stadium, they pushed into the Leinster half and were accurate with their possession and ruck speed.
Then, a penalty was given after Leinster were too slow too roll away at a ruck.
Cometh the hour, cometh the man.
The Kingspan Stadium fell so silent, you could hear a pin drop, as John Cooney placed down the tee and went through his ritual.
As cool as ice, Cooney was the man for the moment.
His right boot swung perfectly into the ball, and as Ulster's supporters seen the trajectory - they shouted in jubilation.
A kick that secured a place in the top eight of the URC and play-off rugby.
Re-grouping quickly after the points, Ulster kept the ball well from the restart and saw out the clock, with Nathan Doak bringing on he final whistle with his kick out of play.
Kingspan Stadium rocked in celebration.
A night to remember in Belfast, a comeback to remember.
Richie Murphy's men had done the double over Leinster in the URC and can now look forward to a final regular season round against Munster at Thomond Park.
FT: Ulster 23-21 Leinster
Player of the match - John Cooney
As reliable as ever from the tee, and aggressive in his play the whole night with his quick balls, kicking and tackling. He grabbed the moment, and made the difference on the night.
Reaction
Post-match with the boss 👊 pic.twitter.com/ziQmk6sTm4
— Ulster Rugby (@UlsterRugby) May 18, 2024
Gallery
Scorers
Ulster (23)
TRY: Herring, Stockdale
CON: Cooney x2
PEN: Cooney x3
Leinster 21
TRY: Ngatai, Foley, Ala'alatoa
CON: Byrne X3
Teams
Ulster
(15-9) Ethan McIlroy, Mike Lowry, Will Addison, Stuart McCloskey, Jacob Stockdale, Billy Burns, John Cooney;
(1-8) Eric O’Sullivan, Rob Herring, Tom O’Toole, Kieran Treadwell, Alan O’Connor (C), Cormac Izuchukwu, David McCann, Nick Timoney.
Replacements: Tom Stewart (Rob Herring), Andy Warwick (Eric O'Sullivan), Scott Wilson (Tom O'Toole), Harry Sheridan (Kieran Treadwell), Matty Rea (Cormac Izuchukwu), Nathan Doak (Billy Burns), Stewart Moore (Ethan McIlroy), Jude Postlethwaite (Stu McCloskey).
Leinster
(15-9) Keenan; T O'Brien, J O'Brien, Ngatai, Russell; Byrne, Foley;
(1-8) Healy, Kelleher, Ala'alatoa; Deeny, Ryan (C); Deegan, Connors, Conan.
Replacements: McKee, Milne, Clarkson, Molony, Penny, McGrath, Prendergast, Brownlee.