Leinster (6) 9
Pens: Sexton 3
London Irish (6) 12
Pens: Hewat 2, Lamb 2
O'Driscoll's Leinster were left to rue a last-minute infringement
A last-minute penalty from Ryan Lamb earned London Irish a thrilling victory over defending champions Leinster in their Heineken Cup opener in Dublin.
A Mike Ross infringement allowed the replacement fly-half to slot over his second penalty from 40 metres.
A tense first half saw the sides level at 6-6 with two penalties apiece from Jonathan Sexton and Peter Hewat.
The Exiles edged in front in the second half, but Sexton levelled in the 75th minute before Lamb's game-winning kick.
Boos echoed around the RDS Stadium when referee Romain Poite blew his whistle following what looked like a high tackle from London Irish number eight Chris Hala'Ufia on Leinster captain Brian O'Driscoll deep inside the Exiles 22 in the final minute.
But the Leinster appeal was ignored by the French official, leaving the travelling Exiles fans to celebrate in the stands following Lamb's superb kick in the final minute.
The former Gloucester fly-half replaced Chris Malone in the 55th minute and made an immediate impact, nailing a penalty to give Irish a 9-6 lead.
A scrum infringement from Hala'Ufia allowed Sexton to level the scores with five minutes left to set up an enthralling finale.
But with neither defence yielding, it was left to Lamb to seal the victory with one minute left on the clock.
Leinster, who comprehensively thrashed 2009 Heineken Cup semi-finalists Munster 30-0 last week, began confidently.
A surging run from captain O'Driscoll set up the first of Sexton's penalties inside the first minute.
Irish began to settle, with Australian full-back Hewat levelling the scores, although the visitors had Malone to thank for a try-saving tackle on winger Luke Fitzgerald.
A tense first half turned into blows in the 26th minute when Leinster number eight Jamie Heaslip and Irish hooker David Paice were sin-binned for a touchline fracas, while Exiles lock Nick Kennedy alleged flanker Shane Jennings had gouged him.
Sexton missed an opportunity to extend Leinster's lead, allowing Hewat to level the scores with his second penalty just before half-time.
Lamb's introduction changed the complexion of the match, landing a superb 66th-minute penalty from distance.
Leinster stormed back with a series of attacks, Fitzgerald denied in the left corner by Hewat and the industrious Steffon Armitage.
Sexton did boot the champions level, but after Ross was adjudged to have kicked the ball from an offside position, Lamb controlled his nerves to kick the match-winning penalty with seconds remaining.
Leinster coach Michael Cheika:
"It was a bit of an arm wrestle at times and we had no flow, especially in the ruck area. The French referee had a different interpretation at the ruck and that created a stop-start nature with lots of penalties.
"At the end of the day though we have to take responsibility for our own performance. London Irish are a good team. I think the result was a combination of them being good and us being poor."
London Irish coach Toby Booth:
"We won a game in the backyard of a very difficult place to come against the defending champions that are in good form.
"We've snuck it at the end and I think the enthusiasm with which the boys celebrated is purely out of respect for Leinster because they know how hard it is to come here and get anything."
Leinster: I Nacewa; S Horgan, B O'Driscoll, G D'Arcy, L Fitzgerald; J Sexton, E Reddan; C Healy, J Fogarty, M Ross, L Cullen, capt, N Hines, K McLaughlin, S Jennings, J Heaslip.
Replacements: B Jackman, R McCormack, CJ van der Linde, M O'Kelly, S O'Brien, S Keogh, S Berne, R Kearney.
London Irish: P Hewat; T Homer, E Seveali'i, S Mapusua, S Tagicakibau; C Malone, P Hodgson; C Dermody, D Paice, F Rautenbach, N Kennedy, B Casey, D Danaher, S Armitage, C Hala'Ufia
Replacements: D Coetzee, D Murphy, P Ion, A Perry, R Thorpe, J Lennard, R Lamb, A Lalanne
Referee: Romain Poite (France)