The bonus-point win was based on an early 22-0 lead for the Irish side with a penalty try and scores from Brendan Macken and Jack McGrath doing the damage.
Simon Easterby’s men hit back with Gareth Davies and Gareth Maule both crossing the try line but Eoin Reddan and South African Zane Kirchner sealed the victory, rendering Adam Warren’s late effort a mere consolation.
Ian Madigan’s penalty put Leinster into an early lead before Macken went over for the first try of the day.
Kirchner then went close to adding a second but after crossing the line, the ball was adjudged to have been held up.
Nonetheless, Leinster demolished the Scarlets scrum from the reset with a penalty try being awarded.
Madigan converted and it wasn’t long before prop McGrath piled over under the posts. The extra two were added again and at this stage, the Scarlets were in trouble.
Leinster lock Leo Cullen was sin-binned for three repeated offences and although the Scarlets failed to hit back immediately, they did so at the start of the second half.
Scrum-half Davies dived over but No. 8 Sione Timani was then yellow carded for tripping Lote Tuqiri.
Being a man down didn’t hinder Easterby’s side though and a breakaway effort narrowed their deficit. Jordan Williams and Warren led a counter-attack which ended with Maule diving over.
Remarkably, there was still time for three more scores as Reddan and Kirchner added to Leinster’s tally and Warren scored a third for the Scarlets in injury time.
