Back row Declan Danaher crossed in the first half but it was the superb Hewat who made the biggest impression on the scoreboard with five penalties, landing him the man-of-the-match award.

Perpignan's response came from South African Percy Montgomery who booted nine points with his side failing to capitalise on two dominant spells in the first half.

Irish, featuring in the competition's knockout stages for the first time, will discover their semi-final opponents tomorrow when Toulouse and Cardiff meet.

And their confidence for the last four showdown at Twickenham will have been sent soaring following their comprehensive victory over one of the Heineken Cup's big guns.

For a match that was simmering with bad blood following two fractious encounters between the rivals in the group stage, there was little evidence of needle.

The only flashpoint arrived in injury time when Perpignan blindside Viliami Vaki was sent off for punching Steffon Armitage amid a large brawl.

Irish focused on using their greater mobility to tip the scales in their favour while Perpignan's most potent weapon was their pack, but as in the group stages it was the Exiles who came out on top.

Perpignan, who were frustrated by a try-saving tackle from Paul Hodgson on Adrien Plante, wilted in the second half with Hewat's superb kicking from hand frequently pinning them back.

Montgomery landed a dubious early penalty awarded for Topsy Ojo's high tackle on openside Ovidiu Tonita as Perpignan sought to exploit a turnover.

It was harsh on Irish who had started brightly, using their high-tempo game plan to stretch the French club.

A five-metre scrum looked ominous for the Exiles, however, as Henry Tuilagi piled forward but number eight Phil Murphy was on hand to snaffle up the ball when it popped loose.

Perpignan's aggressive defence was doing an effective job of keeping the home side shackled and they also made major inroads with the ball in hand.

Their pack was doing the telling damage and it took a well-timed interception by Sailosi Tagicakibau to prevent them capitalising on a 20th-minute overlap.

Two quickfire penalties from Hewat nudged Irish ahead but Montgomery added a second of his own to level the score.

When they had possession, Irish attacked with greater speed and it was one well-worked offensive that enabled them to reclaim the lead in the 31st minute.

Bulldozing centre Seilala Mapusua made the initial inroads and when the ball was recycled Mike Catt chipped into the corner where back row Danaher outjumped Christophe Manas to touch down.

But the try did not come without cost as Catt, who had taken a heavy knock earlier in the game, limped off with substitute Shane Geraghty coming on.

It was Geraghty's first match since suffering a knee injury that ruled him out of England's RBS 6 Nations campaign.

Perpignan winger Plante was denied a certain try by a last-ditch intervention from Hodgson but a penalty from Montgomery left them with some reward.

The opening 10 minutes of the second half were far more cagey, with both teams looking to gain a foothold with the boot.

But the pendulum was swinging Irish's way with two more penalties from Hewat confirming their superiority as they surged 17-9 ahead.

When the Australian's fifth penalty sailed over in the 63rd minute Perpignan looked dead and buried and they were unable to make any impression.

An injury-time brawl that resulted in Vaki's red card was the final act of a memorable afternoon for Irish.

London Irish Try: Danaher; Pens: Hewat (5)

Perpignan Pens: Montgomery (3)

London Irish: Hewat, Ojo, Tiesi, Mapusua, Tagicakibau, Catt, Hodgson, Dermody, Paice, Rautembach, Kennedy, Casey, Danaher, S. Armitage, P. Murphy.
Replacements: Lea'aetoa, Coetzee, Hudson, Thorpe, D. Armitage, Richards, Geraghty.

Perpignan: Montgomery, Manas, Marty, Grandclaude, Plante, N. Laharrague, Cusiter, Chobet, Tincu, Mas, Alvarez, Hines, Vaki, Tonita, Tuilagi.
Replacements: Guirado, Freshwater, Bozzi, Porcu, Durand, Hume, Chouly.

Referee: Alain Rolland (Ireland)