The French visitors bounced back from their loss in Paris last week with a surprise victory in the Pool Two clash at the Madejski Stadium.
Racing wing Sereli Bobo crossed for a try and fly-half Jonathan Wisniewski kicked 20 points with three penalties, three drop goals and a conversion to settle the Pool Two clash.
Hooker David Paice got a try for London Irish with the last move of the game and wing Tom Homer kicked 14 points with four penalties and a conversion, but the hosts fell short.
Just two games ago, the Exiles were all but out of the competition after a one-point home loss to Edinburgh and a 24-18 defeat at Cardiff Blues left them with just two points.
However, last week's superb and unexpected four-try, bonus point victory over Racing brought Toby Booth's side right back into the mix, only for them to fall over again at home.
Booth made two changes in the backs from last week's starting line-up, with Tom Homer coming off the bench to the wing and Topsy Ojo going in the other direction.
Adrian Jarvis came in at fly-half for Dan Bowden, who was injured in last week's game. Up front Ofisa Treviranus switched from flanker to number eight, with Jamie Gibson in at number seven.
Metro, meanwhile, made eight changes in their starting 15 from the match in Paris.
And, inside the first four minutes, after ex-England scrum-half Paul Hodgson had led them out to mark last week's 200th appearance, the Exiles carried on where they left off.
Racing's first clearance kick on their own line was half charged down before the French side gave away a penalty 55 metres from their line, Homer stepping up and hammering it over the posts from inside his own half.
It was a bright start by the Exiles, but Racing, with little to play for in the group, were in no mood to just lie down.
The first period was a kicking contest as Homer landed three more penalties, while Wisniewski punished persistent home errors with points.
Irish gave away two kickable shots at goal for Wisniewski to slot over from ruck errors, and, when Racing's powerful pack, led by France international and former Sale star Sebastien Chabal, got within range, 26-year-old Wisniewski twice dropped goals to give the visitors a 15-12 half-time lead.
Irish needed tries to secure another bonus point, but it was Racing who seized the initiative in the second half.
Wisniewski dropped a third goal after 50 minutes before the opening try of the match came when Bobo kicked on a dropped ball from an Irish player and sped away under the posts to make his stand-off's conversion easy.
At that point, Irish were bottom of their pool and, although they mounted a fightback towards the end, the dominant Racing pack kept them at bay in the set piece.
Wisniewski was sent to the sin-bin for 10 minutes late on after a deliberate knock-on and Irish finally crossed right on the whistle.
Paice was put through a gap five metres from the visitor's line and Homer converted to reduce the deficit to six points and give the hosts a losing bonus point and keep their faint quarter-final hopes alive.
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