Former Champions Munster set up a Heineken Cup Semi-final against Clermont Auvergne when they beat Harlequins 12-18 at the Twickenham Stoop.
The Irish province reached the semis for the first time since 2010 and in so doing avenged a Challenge Cup semi-final defeat to the Londoners two years ago.
In a brutal encounter Munster fly-half Ronan O'Gara outscored Quins' Nick Evans by six penalties to four to maintain Irish hopes of continuing their recent domination in this competition.
Irish sides have won five of the last seven editions of the European Cup and following Ulster's 27-16 defeat at Saracens on Saturday, Munster remain the last representatives from the Emerald Isle.
The first period was a war of attrition with neither side coming close to breaching their opponents' tryline.
Harlequins went into the break 9-6 ahead with missed kicks at goal proving decisive.
Former All Black fly-half Evans was first to misfire but O'Gara hooked two kicks wide before finding his range.
The hosts had their noses just in front but that lead was turned on its head within six minutes of the restart.
Quins captain Chris Robshaw was penalised for offside and O'Gara made no mistake from his kick.
Flanker Peter O'Mahony then robbed line-out ball from George Robson deep in Harlequins' 22 and the hosts infringed handing O'Gara a kick straight in front of the posts.
Munster had steppped up the intensity and Harlequins simply couldn't cope, giving away another penalty in front of the sticks that O'Gara, who now couldn't miss, slotted over for a 15-9 lead.
O'Gara kicked a fifth straight penalty without reply before Harlequins started to wrest some control back from their Irish visitors with a scything break through the midfield from winger Tom Williams.
That set up an Evans penalty but Munster then pushed the hosts deep back into their own territory and controlled the ball to see out a deserved victory.
Favourites Clermont booked their last four spot on Saturday when they ousted fellow French Top 14 side Montpellier 36-14.
The final quarter-final sees twice former winners Leicester travel to Top 14 leaders Toulon.
Final Score Harlequins (9) 12 Munster (6) 18
Pens: Evans 4
Pens: O'Gara 6
Referee: Jerome Garces (Fra),
Assistant ref: Marius Mitrea (Ita),
Assistant ref: Laurent Cardona (Fra),
TMO: Iain Ramage (Sco),
Citing Commissioner: Yves Thieffine (Fra)
Harlequins: Mike Brown; Tom Williams, George Lowe, Tom Casson, Ugo Monye; Nick Evans, Danny Care; Joe Marler, Rob Buchanan, James Johnston, Ollie Kohn, George Robson, Maurie Fa'asavalu, Chris Robshaw (c), Nick Easter.
Replacements: Joe Gray, Mark Lambert, Will Collier, Charlie Matthews, Tom Guest, Karl Dickson, Ben Botica, Matt Hopper.
Munster Rugby: Felix Jones; Denis Hurley, Casey Laulala, James Downey, Simon Zebo; Ronan O'Gara, Conor Murray; Dave Kilcoyne, Mike Sherry, BJ Botha, Donnacha Ryan, Paul O'Connell (c), Peter O'Mahony, Tommy O'Donnell, James Coughlan.
Replacements: Damien Varley, Wian Du Preez, Stephen Archer, Donncha O'Callaghan, Patrick Butler, Cathal Sheridan, Ian Keatley, Ivan Dineen.