The two-time European champions produced a devastating second-half performance, led by captain Paul O'Connell and their dominant back row, to kill off any hopes Harlequins had of a victory.

Harlequins had led 9-6 at the interval, courtesy of three Nick Evans penalties, but they were hit by a ferocious Munster onslaught in the second period.

O'Gara, back at the venue where he played his first Heineken Cup match 14 years ago, kicked four penalties in a devastating 16-minute spell that Munster spent camped in Harlequins territory.

Munster conceded 50 points to Glasgow in the RaboDirect PRO12 last weekend but displayed all their European pedigree to down the English champions and reach a remarkable 10th Heineken Cup semi-final.

There were plenty of performances that will have caught the eye of British and Irish Lions coach Warren Gatland, particularly O'Connell's towering display.

This was Harlequins' first Heineken Cup quarter-final since the Bloodgate scandal erupted at the same stage of the competition four years ago.

Harlequins are a better side and a better club than they were in 2009 but their wait for a debut Heineken Cup semi-final appearance goes on.

Their boss Conor O'Shea had described this as the "biggest occasion" in the club's history and it was some occasion, with The Stoop packed to the rafters.

Harlequins were led out by George Lowe on his 100th appearance and the centre was almost over inside the first minute but he was beaten to Tom Casson's grubber kick by Dennis Hurley.

On their own ball, the Harlequins scrum was dominant led by the Samoan tighthead James Johnston who earned an early penalty, which Evans converted.

O'Gara had a chance to respond but hit the post with his first penalty shot of the day as Munster began to control the territorial battle, but they could not break down the spirited home defence.

When a punt upfield from Munster full-back Felix Jones skipped dead, Harlequins had the scrum and Johnston again went to work on loosehead Dave Kilcoyne to win another penalty.

Evans pushed his 45-metre strike wide but soon extended Harlequins' lead with a successful shot at goal as the hosts punished some careless Munster forward play.

Munster's driving game had been effective but a spilled ball allowed Harlequins to counter-attack through Evans and then Nick Easter before Donnacha Ryan was penalised for not rolling away.

O'Gara then missed again, failing to reward an effective rolling maul, but when Ugo Monye was wrapped up in a tackle by O'Connell it gave Munster a strong attacking platform.

Munster moved the ball wide right to Casey Laulala who got outside Tom Williams. The ball went loose from his offload, Harlequins were penalised in the scramble and O'Gara slotted his first kick.

Harlequins responded immediately. O'Connell spilled the kick-off, Johnston again proved too strong for Kilcoyne in the scrum and Evans kicked his third penalty.

Easter and then Danny Care looked to trouble the Munster defence but the Harlequins scrum-half was caught holding on and it was the Irish province who finished the half with their tails up.

O'Connell had defenders virtually hanging off him as he marched towards the Harlequins 22, eventually earning the penalty which O'Gara kicked.

If the score was tight at half-time, it was Munster who increasingly had the most influential players and that told in the 16 decisive minutes after the interval.

O'Gara kicked four penalties in that time as Munster, playing with much greater urgency, dominated possession and took a match-winning grip on the game.

From the moment Peter O'Mahoney executed an imperious line-out steal five metres from the Harlequins line, Harlequins could not relieve the pressure.

O'Gara kicked Munster into a 12-9 lead and Harlequins began to lose their composure, with poor passes from Evans and Williams landing Maurie Fa'asavalu in trouble.

O'Gara extended the lead with his fifth penalty as Munster began to get dangerous runners Laulala and Simon Zebo into the game for the first time.

Harlequins full-back Mike Brown was floored by a shuddering tackle from James Downey as Munster kept their foot on Harlequin throats, with O'Gara opening a nine-point lead.

That was game over.

Harlequins tried to gain a foothold and Evans replied with a penalty after a searing break from Williams and then Tom Guest but Munster's defence simply suffocated them out of the contest.