Leinster put themselves in prime position for a Magners League play-off spot with a swashbuckling derby win over Ulster.

The Heineken Cup semi-finalists stung their provincial rivals with four tries in the opening 36 minutes for the fastest bonus point of the current league campaign.

Richardt Strauss, Luke Fitzgerald, Shane Horgan (2) and Heinke van der Merwe were the try scorers for Leinster who have vaulted above Ulster into second place.

Six days after their heartbreaking European exit, Ulster showed great resolve with tries from Pedrie Wannenburg and Craig Gilroy to almost snatch an unlikely losing bonus point.

Leinster got off to a dream start when hooker Strauss burrowed over after just 69 seconds.

Ulster were caught for numbers when Jonathan Sexton dinked a kick towards the left corner, Fitzgerald and Adam D'Arcy battled for the ball and when it went loose, Eoin O'Malley's flicked pass sent Strauss diving over.

Sexton drilled his conversion attempt narrowly to the left, and Ulster gathered themselves for a swift response. They earned a penalty on the ten-metre line which Ian Humphreys knocked over with aplomb.

But Leinster exerted their authority again, with Sexton drawing two players to put Isa Nacewa through a gap on the left and he passed for Fitzgerald to ghost over in the corner.

This time Sexton pushed the conversion to the right and wide. It did not matter as Leinster were brimming with confidence and had their third try on the board by the 13th minute.

The forwards pummelled away before Fergus McFadden's looping pass gave Horgan enough time and space to barge over in the right corner. Sexton added the extras for a 17-3 advantage.

Ulster's best attacking phase could have led to a try and a yellow card for Leinster flanker Shane Jennings, but they had to settle for a close range kick from Humphreys.

Ulster again built through the phases, bringing D'Arcy and Andrew Trimble into play out wide, and Humphreys' third successful penalty punished Stan Wright for a binding offence.

Sexton nailed a penalty in response and referee Pascal Gauzere reached for his yellow card when Ulster number 8 Wannenburg tackled Fitzgerald without the ball, with Leinster pressing for another try.

That bonus point score came a minute later, the home forwards barging up in a maul which saw prop van der Merwe touch down by the posts.

Sexton's conversion was cancelled out by a monster penalty from Ruan Pienaar in injury-time, while Humphreys was treated for a knock, leaving it 27-12 at the break.

Ulster's bad night was summed when they blew an overlap on the right, as the second half resumed. Robbie Diack fumbled while Kevin McLaughlin and Leo Cullen were down receiving medical treatment.

Then D'Arcy and Pienaar were both caught out when a hungry Horgan attacked down the right, deftly collected his own chip and breezed over for an instinctive 44th minute try.

Sexton converted for a 34-12 scoreline and Joe Schmidt was soon emptying his bench. Brian McLaughlin did likewise as the short turnaround between games took its toll.

One of Ulster's replacements, Willie Faloon, helped his back row colleague Wannenburg wrestle his way over for a try that the visitors' efforts deserved. The conversion was successful from Pienaar.

Their hopes of a first win in Dublin since 1999 may have been dashed, but Ulster ploughed on. After Leinster replacement Sean O'Brien was sin-binned and McFadden missed a long range penalty, the hardworking Gilroy fended off Jennings to grab his eighth league try.

It was a well-taken score from the impressive young winger. Pienaar converted and Leinster's forwards sealed off any threat of a comeback with some good work around the fringes.

The defeat dents Ulster's semi-final ambitions, but they remain third in the table with just two rounds remaining.

Speaking afterwards, Leinster flanker Shane Jennings, who made his return from a two month injury lay-off, said: "It was good from the perspective of getting the bonus point, but there were some defensive errors on our part that we'll have to tighten up on.

"It was a strange game. I felt our set piece was good overall but we weren't as strong defensively.

"It was good to get back (after being out with the knee injury) because it had been a frustrating last couple of weeks, personally speaking.

"It was good though to see the team do so well over that time. We have an exciting few weeks ahead, but our concentration is on Aironi next weekend."