A last-minute drop-goal from Ireland and Leinster fly-half Jonathan Sexton saw off a sprited Connacht 17-14 at the RDS on Saturday.
This was top versus bottom in the Magners League but Connacht really turned the tables on a much-fancied Leinster side, giving themselves every chance of recording their first victory in Dublin since 2002.
Winger Fionn Carr, a former Leinster Academy player, showed his home province what they are missing with two excellent tries in the opening 22 minutes.
Ireland and Lions star Rob Kearney touched down to close the gap to 14-8 at half-time, and Sexton won the battle of the out-halves in the second period by landing two penalties and the decisive 79th-minute drop goal.
It was cruel luck for Connacht for whom Ian Keatley missed two drop goal efforts and his fellow replacement Sean Cronin was inches away from scoring a try, with seven minutes remaining.
This inter-provincial derby was just 83 seconds old when Carr crashed over for his first try. Connacht's danger man, who has yet to feature for Ireland at Test level, caught Shaun Berne, Girvan Dempsey and Kearney all off guard as he cut in from the left and sprinted clear and under the posts.
New Zealander Miah Nikora added the conversion and although Connacht conceded a raft of penalties in the opening quarter, Leinster's sole reward was a fifth minute strike from Sexton.
The hosts' territorial dominance came to nothing and they were rocked again when Dempsey was robbed at a ruck and Carr linked with Nikora before gleefully dotting down to the left of the posts.
Nikora, with a considerable wind behind him, fired over a brilliant conversion to increase the pressure on Leinster who were missing the likes of Brian O'Driscoll, Gordon D'Arcy, Cian Healy and Jamie Heaslip.
Michael Cheika's men looked back on track, six minutes later, as Kearney dummied his way past Brian Tuohy and Aidan Wynne, just outside the visitors' 22, on the way to a well-taken try.
Sexton failed to convert and also pushed an injury-time penalty effort to the right of the posts, as Connacht held a six-point advantage at the break.
With Leo Cullen and his forward colleagues getting more of a grip on matters up front, Leinster reduced the arrears to three points thanks to Sexton - although the recent Six Nations debutant's radar was off in the 50th minute when he had a chance to level.
Drama followed as, with twelve minutes to go, Connacht lost full-back and captain for the day, Gavin Duffy, to the sin-bin for a punch, spotted by the touch judge. The television replay showed Duffy tangling with Fergus McFadden, but there was little or no evidence of a punch.
With the electric Carr continuing to threaten, Connacht looked like they might survive and a rousing breakaway almost had the perfect ending as Cronin sped towards the line. However, a desperate tackle from Dempsey felled the hooker, and Kearney got his arms under the ball to prevent the try.
Keatley then missed the easier of his two drop goal shots, from in front of the posts, and although Leinster scrum half Eoin Reddan did likewise at the other end, his half-back partner Sexton kept his nerve to coolly slot a 40-metre drop and dash Connacht's hopes at the death.
Pen: Sexton 3
Tries: Carr 2
Con: Nikora 2
Leinster: 15 Rob Kearney, 14 Girvan Dempsey, 13 Fergus McFadden, 12 Shaun Berne, 11 Isa Nacewa, 10 Jonathan Sexton, 9 Eoin Reddan, 8 Stephen Keogh, 7 Shane Jennings, 6 Rhys Ruddock, 5 Malcolm O'Kelly, 4 Leo Cullen (capt), 3 Mike Ross, 2 John Fogarty, 1 Stan Wright.
Replacements: 16 Bernard Jackman, 17 CJ van der Linde, 18 Devin Toner, 19 Paul Ryan, 20 Paul O'Donohoe, 21 Eoin O'Malley, 22 Simon Keogh.
Connacht: 15 Gavin Duffy (capt), 14 Brian Tuohy, 13 Aidan Wynne, 12 Keith Matthews, 11 Fionn Carr, 10 Miah Nikora, 9 Frank Murphy, 8 George Naoupu, 7 Johnny O'Connor, 6 Mike McComish, 5 Bernie Upton, 4 Mike McCarthy, 3 Robbie Morris, 2 Adrian Flavin, 1 Brett Wilkinson.
Replacements: 16 Sean Cronin, 17 Jamie Hagan, 18 Michael Swift, 19 Andrew Browne, 20 Conor O'Loughlin, 21 Ian Keatley, 22 Troy Nathan.
Referee: Jerome Garces (FFR)
Assistant Referees: S McDowell (IRFU), J Quinn (IRFU)
TMO: Peter Ferguson (IRFU)