London Irish are one win away from a first quarter-final appearance in the Heineken Cup after Richard Thorpe scored a hat-trick of tries to kill off an unlikely Dragons comeback.

The Exiles looked out of sight at 34-5 up early in the second half after two tries from Thorpe and one each for Eoghan Hickey, Shane Geraghty and Peter Hewat at the Madjeski Stadium.

But they took their foot off the pedal and missed the vital tackles to let the Welsh side - who had two players sin-binned - back into the game.

Ben Daly, Gareth Wyatt and Ceri Sweeney touched down to add to Richard Fussell's early score before normal service was resumed when Irish shoved Thorpe over at a close range line-out five minutes from time.

This European Cup pool is tightly balanced with Irish leading Perpignan by a single point going into the penultimate round of games.

The only way they could make certain of a quarter-final appearance was to claim bonus point victories against both the Dragons and against Treviso next Saturday.

They scored the four tries needed within half an hour against a Welsh side short of a dozen leading players.

Hewat's early penalty was followed by a 13th-minute try from No8 Thorpe, who broke four tackles from a standing start to score on the right.

Five minutes later Hickey added a second by following up a Peter Richards kick and two conversions by Hewat left the Dragons playing for pride long before Paul Emerick was sin-binned for killing the ball.

While the centre was off the field, Geraghty dodged his way through a midfield gap for a third score before winger Topsy Ojo handed a gift try to Fussell.

It interrupted the onslaught only briefly with Hewat clinching the bonus point five minutes before the break when he cantered in from 30 metres.

Lock Peter Sidoli became the second Dragons player to be yellow-carded just before the interval and Thorpe added his second try soon after it to put Irish seemingly out of sight.

They tried to throw it away by conceding three soft tries in the next 20 minutes, though.

Hooker Daly somehow eluded Sailosi Tagicakibau to collect a cross-kick and score on the right, replacement Wyatt dotted down a simple overlap on the hour and then Sweeney eluded more half hearted tackles 15 minutes from time.

The fly-half's two conversions left his side only 10 points down before Irish opted for a more straightforward approach to shove Thorpe over in the corner.