The Exiles travelled to the Stade Yves Du Manoir on the back of successive European defeats to Edinburgh and Cardiff Blues, leaving them with a mountain to climb to match their semi-final showing from 2008.
But a bonus-point win against the big-spending French outfit has put them right back in the mix ahead of the return fixture this time next week.
Tries from Jonathan Spratt, Joe Ansbro and a brace from Adam Thompstone gave Irish only a second-ever Heineken Cup success on French soil, leaving Booth an understandably happy man.
"It was always going to be very tough coming here. Everyone talks about winning your home games in the Heineken Cup but the key to surviving is to win on the road," said Booth.
"We've done that and we've picked up a bonus point. It's the first step to keeping our Heineken Cup hopes alive.
"I couldn't have asked for more, both in terms of the result and the performance. To go up against a French institution like Racing - with the budget, resources and quality of players they have - and to win is a great result for us.
"It's something we can hang our hat on going forward. It's shown our very young side that we can compete with the best."
Irish trailed 14-3 after 22 minutes in Colombes but Spratt's quick response to a Sereli Bobo try narrowed the gap to four points at the interval before an inspired four-minute spell effectively sealed a famous triumph.
Ansbro's fine individual try in the 54th minute was followed by an equally impressive effort from Thompstone in the 58th as Irish surged 13 points clear, with a 45-metre intercept from the hard-working left wing then sealing the win with seven minutes left.
But Booth, who never feared his side would be blown away despite their disappointing start, insists Irish will not be resting on their laurels going into Saturday's clash at the Madejski Stadium.
"I never thought we were out of it. Our biggest loss this season has been six points and that was when we were down to 14 men for 70 minutes against Cardiff in the last round. We've always been competitive," added Booth.
"Racing will come to us next week hurting, though. They'll be devastated by this loss and next week's game is a potential hiccup for us.
"Consistency is absolutely key for us now. This win will mean nothing if we don't back it up it next week".
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