Connacht turned the Magners League table on its head to dish out a 14-point defeat to leaders Leinster and boost their chances of qualifying for the Heineken Cup for the first time.
The Sportsground erupted at the final whistle as tries from Brian Tuohy, Michael Swift and Troy Nathan helped Connacht to only their second win over Leinster since 2004.
They won the equivalent fixture last season needing the boot of Ian Keatley to see them home at 19-18, but this result shows just how far Michael Bradley’s men have progressed even since then.
They are now off the bottom of the league table and level on 26 points with Ulster, the side they are challenging for Ireland’s final qualification spot in the Heineken Cup. Intriguingly, Ulster host Connacht in the Magners League’s final round over the second weekend in May.
This derby tie seemed set up for Leinster to win and guarantee themselves a home semi-final, especially with Brian O’Driscoll returning from a knee injury in a strong starting line-up.
But Connacht, bouncing back from Sunday’s loss to Munster, rose heroically to the occasion with captain John Muldoon and full-back Gavin Duffy simply immense.
Connacht weathered an early Leinster storm before a sidestepping break from Duffy put Leinster in retreat and had Nathan’s pass out of the tackle found Tuohy in support on the right, the winger looked set to score.
Connacht had been outmuscled by Munster in the forward exchanges on Sunday, but they were much quicker and sharper at the breakdown in this clash, frustrating Leinster time and again.
Big South African CJ van der Linde had the measure of Ronan Loughney in the early scrums, but Connacht avoided a penalty try and secured two priceless turnovers in their 22, the second after Kyle Tonetti had broken the defensive line amid a flurry of Leinster attacks.
As O’Driscoll and Shaun Berne probed off Eoin Reddan’s feeds, a territorially-dominant Leinster continued to batter away at Connacht’s stubborn resistance. Some more committed defence saw prop Stan Wright held up over the try-line.
A quick turnover and kick downfield lifted the siege and from the following lineout, Connacht struck from the opening try.
The ball was swept out to the left where Aidan Wynne, Conor O’Loughlin and Sean Cronin all made yardage before Fionn Carr hurdled a close-in ruck and offloaded for Tuohy to nip in under the posts.
Miah Nikora added the conversion but was off target with a long range penalty into a slight wind as the first half ended at 7-0.
Leinster introduced Jamie Heaslip and Cian Healy for the second half and with O’Driscoll heavily involved, the eager visitors won an early penalty which Berne nudged over.
But Connacht floored the European champions with a tremendous scoring burst.
First, Nikora dinked over a well-taken drop goal and then a thrilling counter attack, sparked by Duffy and continued by some superb offloads, ended with second row Swift, the province’s most-capped player, racing down the left touchline to score in the corner.
Nikora’s conversion added to Connacht’s sudden momentum at 17-3 and though Berne clawed back three points, another scintillating run by Duffy set the westerners up for their third try, cheekily scored by Nathan off a ruck in front of the posts.
Replacement Ian Keatley’s conversion took Connacht’s tally to 24. Leinster needed a break and they got it when Johnny O’Connor, also on as a replacement, was sin-binned for playing the ball on the ground.
Three minutes later, Leinster were awarded a penalty try – as the home front row gave way – and Berne’s drop-kicked conversion closed the gap to 24-13.
But the tireless Muldoon stoked the fires for Connacht once again and when Cian Healy was pinged for not rolling away outside his 22, his old Belvedere school-mate Keatley converted the penalty to put Bradley’s side out of sight.
Speaking after the game, Michael Cheika, who has named a youthful Leinster side for Friday’s visit to Glasgow, said: "Connacht have shown pretty convincingly all season that they are a good side, even in the game up in Dublin when we were lucky to win.
"I don’t think that they’re a bottom of the league team and we know that they have a lot of quality because we have coached a few of them ourselves.
"We lacked an edge in defence. We had a lot of possession but we didn’t use it properly, but in the second half we attacked the line better.
"We worked hard, but it’s another game that we have lost here because we lacked the correct attitude.
"Brian (ODriscoll) finished the game and he’s fine, but Girvan (Dempsey) suffered a neck injury and we just have to manage our resources now with a game coming up against Glasgow on Friday."