After storming into a 22-0 lead in the opening 29 minutes, Leinster were not able to seal the bonus point until nine minutes from time.

First-half tries came from Shane Horgan, Gordon D’Arcy and Shaun Berne, who provided 17 points, before flanker Sean O’Brien grabbed the vital try.

The Scarlets, who were missing injured trio Mark Jones, Matthew Rees and Dafydd Jones, struck back briefly in the second half with a try from Jonathan Davies.

It took just three minutes and 15 seconds for Leinster to cross for their opening try when winger Horgan sped through a gap between Andy Fenby and Daniel Evans and rounded two more tacklers on a 40-metre run to the line.

Berne converted and then added a penalty before Leinster scored two more tries in five minutes.

The lively Isa Nacewa was sent down the wing by Brian O’Driscoll, who the home fans believed should have been in the sin-bin for a high tackle on Evans, before D’Arcy was sent over.

And Leinster cruised into a 22-0 lead after a quick Eion Reddan throw-in was moved quickly across the field for Berne to go over in the corner.

The Scarlets’ intent to get on the scoreboard was obvious after the interval and five minutes into the half Davies forced his way over for a converted try.

That upped the tempo of the contest and more pressure from the Scarlets saw Leinster lock Nathan Hines sin-binned for a professional foul.

Although Leinster re-grouped, they missed a chance of a long-range penalty goal when Rob Kearney fired wide.

The atmosphere was stoked up even more when Sean Lamont was yellow-carded for an illegal challenge on Nacewa, which saw Berne kick Leinster into a 25-7 lead on 56 minutes.

As the match wore on Leinster must have started wondering where the fourth try was coming from, but it duly arrived on 72 minutes when Jamie Heaslip sent O’Brien in for the key score.