Brilliant Williams’ scintillating solo score helped inflict only a second defeat of the season on Aviva Premiership leaders Irish.

And the visitors could have few complaints as Williams’ fellow Lions wing Tommy Bowe claimed an early touchdown before fly-half Dan Biggar kicked four penalties.

Centre James Hook, who moved to 10 when Biggar went off injured early in the second period, added a conversion and penalty, while Irish mustered a Sailosi Tagicakibau interception try, plus 11 points from Ryan Lamb.

The Ospreys, Heineken quarter-finalists in each of the past three seasons, delivered one of their finest European performances.

They were hungry from the start, and the win has put them firmly in quarter-final contention from Pool Three, a punishing group that also includes Irish heavyweights Munster and Jonny Wilkinson’s Toulon.

Irish will also undoubtedly feature at the qualifying business end in January, but it was a sobering night for them at the Liberty Stadium as Ospreys – and Williams in particular – produced some champagne rugby.

The Ospreys made only one change following last weekend’s defeat in Toulon, but it was a notable switch as Wales captain and international back-row forward Ryan Jones packed down at lock alongside Alun-Wyn Jones.

Irish rested full-back Delon Armitage under the England elite player squad agreement, so wing Topsy Ojo replaced him, while head coach Toby Booth handed starts to teenage wing Jonathan Joseph, centre Dan Bowden and number eight Chris Hala’ufia, albeit in the Exiles’ injury-hit position of openside flanker.

Conditions were perfect, and the opening exchanges did not disappoint as the Ospreys took just four minutes to open their account.

Irish struggled to clear the ball through a couple of poor defensive kicks, and there was a ruthless nature about Ospreys’ first attack as Hook sliced through the Irish defence before sending Bowe over.

Biggar could not add the extras, and it was his opposite number Lamb who settled Irish down, booting two penalties in quick succession for a one-point advantage midway through the opening quarter.

Biggar responded with a successful penalty strike from just inside his own half, yet it was with ball in hand that Ospreys continued to cause serious trouble for their opponents.

Williams then set off on a 50-metre run which almost resulted in a try before he fell attempting to hurdle Irish’s last line of defence, and there was no sign of Ospreys taking their foot off the pedal.

Scrum-half Mike Phillips was called back by referee Peter Fitzgibbon after appearing to score a perfectly good close-range try, but with Irish continuing to infringe regularly at the breakdown, Ospreys had more scoring chances as half-time approached.

Biggar duly punished the visitors, kicking three penalties during an eight-minute spell and giving the Ospreys a thoroughly deserved 17-6 interval advantage.

Irish knew they had to open the second-half scoring, and the response came inside a minute when Tagicakibau intercepted Ospreys full-back Lee Byrne’s pass for a 70-metre breakaway score.

Hook, clinging to the Samoan’s shirt for the final 10 metres, tried his utmost to bring him down, but with Lamb adding a touchline conversion, Irish were firmly back in contention.

The Ospreys then lost a groggy Biggar – he was replaced by Scotland wing Nikki Walker, with Hook moving to fly-half and Bowe switching into midfield – as Irish established a foothold in the contest.

Ospreys then gained a fortuitous penalty when Hala’ufia was harshly punished for his heavy-duty tackle on Phillips, but they failed to capitalise on promising field position.

The Ospreys needed a flash of inspiration to get them going again, and it arrived right on cue from the mesmeric Williams.

After receiving the ball 10 metres inside his own half, he kicked cleverly into space, gathered the bounce and then scorched clear without an Irish defender touching him.

It was a try good enough to win any game anywhere, and when Hook booted a 49-metre penalty shortly afterwards, Irish looked down and out at 27-13 adrift.

Lamb’s third successful penalty maintained chances of a losing bonus point, and Ospreys then saw their former New Zealand Test flanker Jerry Collins sin-binned.

But the home side were home and dry, their European campaign up and running in memorable fashion.