The Exiles led 22-10 at half-time, thanks to tries from Tom Homer, David Paice and Chris Hala’ufia, but ended up 31-22 down to the Scarlets, who lie third in Pool Six, by the final whistle.
The turnaround means Irish must pull off a bonus-point win against European champions Leinster at Twickenham on Saturday to clinch automatic qualification to the quarter-finals.
“Irrespective of the mathematics involved, any team that wants to be competitive in any competition has to bounce back from adversity,” said Booth. “I have no problem getting this team up for big games. We have to prepare well for next weekend and try to heal some of the scars.”
In Pool Five, a brace of tries from England wing Ugo Monye could not prevent Harlequins slipping to a fifth successive European loss, a 33-21 defeat to Toulouse. The French club’s bonus-point win secured their place in the last eight.
Northampton wing Chris Ashton completed a memorable week in which he received his first England call-up by scoring in a 34-0 rout of Perpignan.
Ashton’s try was one of four as the Saints closed the gap on Munster. Victory over the Pool One leaders next week will send Northampton into the knockout stage.