A brilliant 113 from just 63 balls, the fastest in the tournament, helps the minnows register the most successful run chase in a World Cup.
O'Brien's record ton helps Ireland beat England in another World Cup shocker
O'Brien, whose older brother Niall played the pivotal hand when Pakistan were beaten at Sabina Park four years ago, obliged by obliterating Matthew Hayden's record for the fastest World Cup century (66 balls), racing to the landmark in just 50 deliveries, with 13 fours and six sixes, one of which was measured at 102m - the longest hit of the tournament.
By the time he departed, run out going for a non-existent second run, they needed just 11 from 11. Trent Johnston, captain back in 2007, nervelessly stroked the first ball he faced through cover, and John Mooney then crowned a magnificent allround performance - four for 62 and 33 not out - by clipping the first ball of James Anderson's final over through midwicket for four.
It was by far the biggest chase in World Cup history, putting in the shade the 312 that Sri Lanka overhauled against Zimbabwe at New Plymouth 19 years ago.
It left England, who had been part of a dramatic tie against India just three days earlier, in a state of shock, and facing three tricky games to avoid yet another underwhelming World Cup campaign.
William Porterfield, the captain, called it the greatest win in Irish cricket history, but it was so much more than that. "Beating England in any sport is fantastic," he said, comparing the feat to the rugby Grand Slam and Irish football's exploits under Jack Charlton. "We've given people back home something to smile about."
The turning point was undoubtedly the batting Power play that Ireland took in the 32nd over. It produced a whopping 62 runs, with the century partnership between O'Brien and Alex Cusack taking just 61 deliveries. By the time a poor bit of running separated them after they had put on 162, just 55 were needed from 51 balls.
Against Bangladesh, they bungled it from a similar situation. This time, there were no such mistakes. There were fortuitous edges and heaves that fell into space, but there were also perfectly decent deliveries smashed to the rope.
"It still hasn't sunk in," said O'Brien afterwards. Myself and Cusy just took a chance and it came off. To score 100 in 50 balls in front of a billion people.under lights against England.what could be better than that?"
Andrew Strauss was left to reflect on another wretched display in the field and with the ball. Graeme Swann finished with three for 47, but the others went for plenty, and catches slipped out of nervous hands. Strauss was one of the culprits, running all the way from cover to mid-off but failing to hang on when O'Brien was on 91.
England's innings had been built around a 91-run opening partnership between Strauss and Kevin Pietersen, both out to needlessly ambitious shots, and a 167-run stand between Jonathan Trott (92) and Ian Bell (81). But with Mooney and Johnston varying pace and length cleverly in the final 10 overs, they lost 6 for 70.
Trott had joined Pietersen and Viv Richards as the quickest to 1000 runs (21 innings) when he got to 64, but his exit with 5.3 overs remaining gave Ireland the opening they needed. O'Brien did the rest. "They go out with an attitude that they've got nothing to lose," said Strauss. "I can't speak too highly of them."
The ICC wants a 10-team World Cup in 2015, and beaming Porterfield spoke of how his side deserved a chance to be part of it. "It could be the death of a lot of teams if it's 10 teams and there's no qualification process," he said.
But after illuminating two successive World Cups with their spirit, surely they must be given that chance.
Andrew Strauss b Dockrell 34
Kevin Pietersen c N O'Brien b Stirling 59
Jonathan Trott b Mooney 92
Ian Bell c Stirling b Mooney 81
Paul Collingwood c K O'Brien b Mooney 16
Matt Prior b Johnston 6
Tim Bresnan c Johnston b Mooney 4
Michael Yardy b Johnston 3
Graeme Swann not out 9
Extras: (1b, 2lb, 20w) 23
Total : (for eight wickets, 50 overs) 327
Fall of wickets: 1-91, 2-111, 3-278, 4-288, 5-299, 6-312, 7-317, 8-327.
Did not bat: Stuart Broad, James Anderson.
Bowling: Boyd Rankin 7-0-51-0 (4w), Trent Johnston 10-0-58-2, Alex Cusack 4-0-39-0 (1w), George Dockrell 10-0-68-1 (5w), John Mooney 9-0-63-4 (1w), Paul Stirling 10-0-45-1.
W Porterfield b Anderson 0
P Stirling c Pietersen b Bresnan 32
E Joyce st Prior b Swann 32
N O'Brien b Swann 29
G Wilson lbw b Swann 3
K O'Brien run out 113
A Cusack run out 47
J Mooney not out 33
D Johnston not out 7
Extras (b 5 lb 16 w 12) 33
Total (for seven wickets; 49.1 overs) 329
Did not bat: G Dockrell, B Rankin
Fall of wickets: 1-0, 2-62, 3-103, 4-106, 5-111, 6-273, 7-317
Bowling: Anderson 8.1-1-49-1 (1w), Broad 9-0-73-0 (2w), Bresnan 10-0-64-1 (2w), Yardy 7-0-49-0 (2w), Swann 10-0-47-3, Collingwood 5-0-26-0
Result: Ireland won by three wickets
FASTEST WORLD CUP HUNDREDS
50 balls - Kevin O'Brien (Ireland) v England, 2011
66 balls - Matthew Hayden (Australia) v South Africa, 2007
67 balls - John Davison (Canada) v West Indies, 2003
72 balls - Kapil Dev (India) v Zimbabwe, 1983
72 balls - Adam Gilchrist (Australia) v Sri Lanka, 2007
FASTEST ALL-TIME HUNDREDS
102 (37 balls) - Shahid Afridi (Pakistan) v Sri Lanka, 1996
147n.o (44b) - Mark Boucher (South Africa) v Zimbabwe, 2006
117 (45b) - Brian Lara (West Indies) v Bangladesh, 1999
102 (45b) - Shahid Afridi (Pakistan) v India, 2005
134 (48b) - Sanath Jayasuriya (Sri Lanka) v Pakistan, 1996
113 (50b) - Kevin O'Brien (Ireland) v England, 2011
124 (53b) - Shahid Afridi (Pakistan) v Bangladesh, 2010
130 (55b) - Sanath Jayasuriya (Sri Lanka) v Bangladesh, 2008
102 (58b) - AB de Villiers (South Africa) v India, 2010
125n.o (60b) - Virender Sehwag (India) v New Zealand, 2009