Harlequins (12) 17
Tries: Amesbury, Robson Con: Jewell
London Irish (5) 32
Tries: Thorpe, Thompstone, Hewat 2 Cons: Hewat, Hickey Pen: Geraghty
Hewat's second-half double helped London Irish take control
London Irish produced a second-half blitz to down Harlequins in the EDF Energy Cup.
Quins raced into a 12-0 lead as George Robson and Charlie Amesbury went over before Richard Thorpe got the visitors on the board just before half-time.
Neil McMillan gave the hosts a 17-5 lead early in the second half but London Irish then cut loose.
Peter Hewat, Adam Thompstone and James Bailey crossed in a six-minute spell and Hewat grabbed a second late on.
London Irish boss Toby Booth:
"Our aim was to win the game by scoring tries and we did that.
"If you are positive in your rugby, you are more likely to come out on top. We will be going to the Ospreys to win, without question."
Harlequins director of rugby Dean Richards:
"This was a good opportunity for some of our young players to show what they can do and we played some good rugby at times.
"We lost the penalty count 16-9 in the end and it cost us dear.
"We lost our discipline and we have to learn about that - but there are some huge positives to take out of the game."
Harlequins: T Williams; Stegmann, Masson, Taione, Amesbury; Jewell, G Williams; Croall, Brooker, Lambert, Percival, Robson, Davies, Mcmillan, Guest.
Replacements: Barry for Taione (56), Malone for Jewell (59), Care for G Williams (59), Elosu for Lambert (56), Rogers for Davies (62).
Not Used: J Evans, Yorke.
Sin Bin: Elosu (61).
London Irish: Hewat; Bailey, Shabbo, Gower, Thompstone; Geraghty, Parker; Dermody, Buckland, Rautenbach, Johnson, Hudson, Danaher, S Armitage, Fisher.
Replacements: Homer for Thompstone (63), Hickey for Geraghty (78), Lalanne for Parker (46), Paice for Buckland (41), D Murphy for Rautenbach (70), Casey for Johnson (70), Thorpe for Fisher (34).
Ref: Greg Garner (RFU).