The Blues still had a slim chance to qualify for the Millennium Stadium semi-final showdown in March if Wasps also suffered a bad day against Saracens. But the Blues failed to reach the final four after a stuttering performance only found them moveinto top gear in the latter stages of the match.
The visitors were the first to draw blood with an early penalty but a quickly taken tap penalty helped Irish captain Ryan Strudwick set up a powerful drive. The pack powered over the line with flanker Paul Gustard touching down and Ross Laidlaw converting.
Wales international Jamie Robinson kicked another penalty but, with the Blues losing flank Robin Sowden-Taylor to the sin-bin, their defence was weakened and the Exiles took advantage.
They took another tap penalty which they mauled over the line with Gustard claiming try number two. Laidlaw then added the conversion before setting up Irish's next score.
The England U21 player, who started just one first team game this season before today, made a break with an impressive turn of speed following a cleanly taken line-out. The No. 10 eased his way through the Blues defence to offload to waiting centre Rodd Penney who was free to touch down.
Laidlaw's 36th-minute conversion put his side 21-6 up but the Blues managed to claw back some points before the break. With Sowden-Taylor back in the line-out, the Blues took control five metres from the Irish line. Scrum-half Mike Phillips took the ball cleanly and dived over to score. Robinson added the conversion.
Blues hooker Gareth Williams added a much-needed injection of excitement after the re-start, finally putting the visitors in the reckoning. Another well-worked line-out catch and drive paid off with the heavyweight forward powering his way over the try line and Robinson's conversion reducing the Exiles lead to just a point.
Replacement Barry Everitt kept Irish's noses just in front with a 64th-minute penalty. Robinson matched his effort but the Exiles secured victory in the 79th minute with a drop goal from England Under-19s player Shane Geraghty.