A brace of tries in less than five minutes halfway through the second period set the Exiles on their way to their first win of the competition as they bounced back from previous defeats to Edinburgh and Cardiff Blues.
Impressive scores from Joe Ansbro and Adam Thompstone turned the game on its head after 54 and 57 minutes respectively before Thompstone claimed his second with seven minutes remaining at the Stade Yves Du Manoir.
Irish trailed 14-10 at the interval after Jonathan Spratt had matched Sereli Bobo's early effort but there was only one side in it after the break.
Having fallen 3-0 behind thanks to a Gaetan Germain penalty at the very first scrum, Dan Bowden should have levelled the scores in Irish's first sustained attack but the New Zealander hooked his simple effort from straight in front of the sticks.
Bowden hobbled off just after the 10-minute mark but the Exiles came close to the game's first try moments later only to be denied by the TV official.
Scotland centre Ansbro burst through Thomas Bianchin and Bobo as Irish moved the ball from left to right but he was adjudged to have lost control of the ball as he stretched for the line.
Delon Armitage put Irish back on level terms with a 14th-minute penalty only for Germain to re-establish the three-point gap two minutes later.
Racing then claimed the game's opening try with the most memorable moment of the half as Bobo powered over five metres in from touch. With Juan Martin Hernandez off the pitch with a blood injury, Jonathan Wisniewski's perfectly weighted kick ahead was collected on the full by Bobo who swapped passes with former Exeter centre Josh Matavesi before racing clear.
Germain failed to add the difficult kick from wide on the left but a Wisniewski drop goal saw Racing open up an 11-point advantage with 22 minutes gone.
But just as it looked like the game was already slipping away from Irish, Spratt scored his first Heineken Cup try to reignite their hopes. The ex-Osprey and Taranaki midfielder was in the right place at the right time after Topsy Ojo picked up replacement fly-half Adrian Jarvis' deft grubber as Irish matched Racing for adventure and enterprise.
Jarvis easily added the extras from close to the posts to cut the gap to 14-10 and it stayed that way until three minutes after the break, when a strong burst from Ofisa Treviranus saw Racing caught offside and allowed Jarvis to cut the gap to a single point.
Irish should have taken the lead for the first time two minutes later but Jarvis pushed a second penalty wide of the right post from 40 metres out and straight in front.
The game then swung permanently in Irish's favour as Ansbro powered through two weak tackles after taking Jarvis' miss pass just outside the 22, before Thompstone cut a delightful inside line from an attacking scrum.
With Jarvis converting both scores, Irish found themselves 27-14 in front approaching the final quarter.
And while Racing enjoyed a sustained period of pressure that only ended 10 minutes after Thompstone's first score, Irish produced a magnificent defensive display, eventually forcing Racing skipper Lionel Nallet to hang on to the ball in the tackle.
With Racing looking short of ideas in the closing stages, Thompstone put the result beyond doubt as he picked off Bobo's hopeful pass just inside the opposition half to race home unopposed and hand his side a maximum five-point tally.
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