The fly-half scored 16 points, including a try, as his side recovered from a slow start at the Millennium Stadium to record a 31-23 bonus-point triumph, with Dan Evans, Ben John and Tom Grabham also crossing over.
That result guaranteed Steve Tandy's men at least a top-four finish, at the expense of Leinster, who were downed 26-10 by rivals Ulster at the Kingspan Stadium on Friday night.
Ben Te'o went over early on for the reigning champions but Iain Henderson's try helped the hosts led 13-10 at the break, before Craig Gilroy's late effort secured the win as Leinster miss out on a top-four place for the first time in 11 years.
Glasgow Warriors remain top of the GUINNESS PRO12 and in pole position for a home semi-final after they overcame a stubborn Connacht to secure a 31-13 bonus-point win at the Sportsground.
A late first-half double from Nikola Matawalu and Stuart Hogg's score gave the Warriors an unlikely 19-6 lead at the break, before Adam Ashe and Tommy Seymour went over to deny the hosts' second-half fightback.
Munster sealed Leinster's fate and secured their own play-off spot, as Tommy O'Donnell celebrated his 100th cap with a try in a 30-19 bonus-point win against Benetton Treviso at Irish Independent Park.
Early scores for Simon Zebo and CJ Stander set the hosts on their way, before Treviso responded through Matt Luamanu, only for O'Donnell's effort and a penalty try to carry Anthony Foley's side to victory.
In the other Judgment Day clash, Scarlets moved into the top six as they overcame an early deficit to record a 29-10 bonus-point victory over Newport Gwent Dragons.
Dragons centre Tyler Morgan touched down early on before tries from Ken Owens and Scott Williams made it 17-10 to Scarlets at the break, with Liam Williams and Harry Robinson crossing over after the interval.
And Edinburgh maintained the pressure on Scarlets for a top-six finish as they continued their recent surge in form with a 37-0 bonus-point win against Zebre at BT Murrayfield on Friday night.
Roddy Grant claimed a try double, with Ross Ford, Stuart McInally and Dougie Fife also dotting down, as the Italians went down to 13 men at one point.
TRY OF THE WEEKEND
Scarlets' Liam Williams lit up the Millennium Stadium on Judgement Day with a piece of virtuosity to set up a stunning try for Scott Williams.
The fullback collected a pass just outside his own 22, chipping over the Dragons' defensive line to gather, burst clear and offload out of the covering tackle to send his namesake over the whitewash.
BREAK OF THE WEEKEND
Despite Nikola Matawalu's try, the Warriors still trailed a resolute Connacht side until Tommy Seymour produce a moment of sublime invention to carve the hosts defence apart.
The winger showed a searing turn of pace to weave his way through various defenders in a narrow channel, before offloading out the back of his hand for Stuart Hogg to cross over.
PLAY OF THE WEEKEND
Ospreys had already pulled off a superbly coordinated backline move to send Tom Grabham over in the corner against Cardiff Blues before they conjured up an even better play.
Attacking from just outside their own 22, Ospreys went wide before Dan Evans broke through and after he was hauled down, his side shifted the ball right at pace to allow Ben John to score.
PLAYER OF THE WEEKEND
Ulster's dynamic flanker Iain Henderson gave another impressive showing at the heart of his side's strong forward display as he was awarded man of the match in their crucial victory over Leinster.
The Ireland international capped a fine evening with a try as he cut a line any centre would be proud of to collect Ruan Pienaar's short pass and dive over.
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