London Irish (29) 41
Exiles number eight Thorpe (front) grabbed a hat-trick of tries
Tries: Thorpe 3, Hickey, Geraghty, Hewat
Cons: Hewat 3, Hickey
Dragons (7) 24
Tries: Fussell, Daly, Wyatt, Sweeney
Cons: Smith, Sweeney
London Irish resisted a Dragons fightback to remain in pole position for a Heineken Cup quarter-final spot.
The Exiles secured a bonus point by the break as tries from Richard Thorpe, Eoghan Hickey, Shane Geraghty and Peter Hewat helped them into a 29-7 lead.
Richard Fussell scored the only try for the Dragons who had two men sin-binned.
Thorpe then added a second try and although Ben Daly, Gareth Wyatt and Ceri Sweeney scored for the visitors, Thorpe's third try saw the hosts home.
Irish will travel to group minnows Treviso next weekend knowing another bonus-point victory will guarantee them a place in the last eight for the first time in their history.
After Hewat's early penalty gave Irish the lead Thorpe grabbed his first try in the 13th minute, breaking four tackles from a standing start to score on the right.
Five minutes later Hickey added a second as he followed a Peter Richards kick, with Hewat adding both conversions.
Geraghty dodged his way through a midfield gap for the third try but with such a healthy lead right winger Topsy Ojo became over confident.
He tried to chip over the cover in his own 22 but failed to clear the on-rushing Dragons and Fussell was the man to profit.
However, Irish soon hit back and Hewat collected the bonus-point try when the hosts secured turnover ball, Richards fed Delon Armitage and he popped the ball inside for Hewat to race over from distance.
Hewat missed the conversion but the Exiles led 29-7 at half-time.
Just before the break the visitors, who had already seen US Eagles centre Paul Emerick sin-binned, lost Peter Sidoli to a yellow card.
When number eight Thorpe grabbed his second in the corner after some sevens-style passing it looked as though the hosts would run riot.
However, with the Exiles ringing the changes, it was the Dragons who belatedly came to life.
Daly picked up Michael Owen's cross-kick to score their second try and Joe Bearman then powered off the back of a scrum, with Emerick's long pass sending Wyatt over on the right to make it 34-17.
When Sweeney sliced through and converted his own try the Welsh side had a bonus point and the gap was down to 10 points.
But the Irish tightened up their game and Thorpe drilled over from close range to complete his hat-trick and kill off any chance of the hosts losing a game they should have won much more comfortably.
London Irish director of rugby Brian Smith:
"Richard Thorpe has been given some opportunities this year and he's really grabbed them. If he was more greedy he'd have scored four.
"But maybe his best position is six or seven [and not number eight]. He's in the Lewis Moody mould - he's very destructive and can maintain that output for 80 minutes every week.
"We had a very good first half but the second was somewhere between poor and appalling. Its a reality check for us and we will shake things up in the camp this week ahead of the Treviso game - we can't be that loose."
Dragons coach Paul Turner:
"You can't get it so wrong defensively in the first half of a game against a side of Irish's quality and expect to win.
"We had an old-style team-talk at half-time and the boys responded to that. At 34-24 down if we'd had scored again it might have been different.
"We ended up with three or four academy players on the field and I was pleased with the second half but we've got to grow up quickly because the first half wasn't exceptional."
London Irish: Hewat, Ojo, D Armitage, Geraghty, Tagicakibau, Hickey, Richards, Dermody, Paice, Lea'aetoa, Kennedy, Casey, Danaher, S Armitage, Thorpe.
Replacements: D. Murphy, Coetzee, Hudson, P Murphy, De Vedia, Hodgson, Tiesi.
Newport-Gwent Dragons: A Thomas, Mustoe, Emerick, Smith, Fussell, Sweeney, W Evans, Black, Daly, Robinson, Hall, Sidoli, Bearman, Harris, Owen.
Replacements: Crawford, Gustafson, A Jones, Walker, Dolman, Wyatt, L Evans.
Referee: P Allan (SRFU)