The two-time winners were always in front following first-half tries from George Robson and Danny Care.
And, despite Felix Jones' try for Munster on the stroke of half-time and a late consolation from Doug Howlett, Quins replacement fly-half Rory Clegg added two second half penalties to seal a famous victory.

Even a red card for number eight Nick Easter, who was twice yellow carded in the second half, could not dampen the celebrations for the Aviva Premiership outfit in Limerick.

Now Quins, who also won the trophy in 2001 and 2004, will face Stade Francais in the final and keep their hopes of qualifying for next season's Heineken Cup alive.

Nick Easter claimed a victory at Thomond Park would be Harlequins' 'biggest' result in their 145-year history and the visitors produced a confident start by taking the lead inside nine minutes.

Robson was left the home faithful stunned after Gonzo Camacho beat Paul Warwick and Doug Howlett down the left flank and as Munster struggled to regroup, the ball was neatly worked across the field to allow the giant lock to score.
Quins fly-half Nick Evans added the conversion and almost set up a second try moments later.

Evans sent through a teasing kick but Keith Earls covered well to deny British Lions team-mate Ugo Monye.

While Evans was influential in open-play, the New Zealander was off-colour with the boot as he missed two penalty chances to extend the lead on 26 and 28 minutes.

The Kiwi then appeared to pick up a knee injury but Quins still grabbed the all-important second try.

With Evans struggling, full-back Mike Brown took over from Evans temporarily at fly-half. He was held up short of the line and, after Evans showed courage to go close, Care burrowed over from close-range.

It was a big call by television match official Giulio De Santis who deemed Care had enough downward pressure to double Quins' try count.

Munster refused to lie down and were back in the contest seconds before half-time when Jones collected Paul Warwick's pass to touchdown before Ronan O'Gara's conversion reduced the deficit to seven points at the break.

The Irish province are on the front foot early in the second half as they go back to basics with replacement Paul O'Connell making a stirring impact off the bench.

French official Romain Poite was forced to send Donncha O'Callaghan and Easter to the cooler following a scuffle on the floor and Clegg hammered home a 49th minute penalty.

The pair returned but Clegg added another invaluable three points shortly before the hour mark.

Quins felt they should have been awarded a penalty try as George Lowe believed he had been tripped by Doug Howlett but the TMO deemed there was no foul play.

Referee Poite then had to disallow two more tries as Howlett received forward passes from Mafi and then O'Connell as Munster were desperate to get back into the match.

Easter was dismissed after the England number eight came in from the side as O'Connell and Varley went close.
Howlett finally crossed with seconds remaining but O'Gara missed the touchline conversion as Harlequins sealed a famous victory to become only the second side – after Leicester – to win a European tie at Thomond Park in the last 16 years.