Despite being hit by the late withdrawal of number eight Scott Quinnell, the Scarlets dominated the opening quarter, only to be denied by some stubborn and well-marshalled Leinster defence.
The Scarlets’ only reward came via two penalties by fly-half Stephen Jones after the visitors were twice penalised for offside in front of the posts.
The Dubliners struck back with the game’s first try on 28 minutes when a strong run by lock Ben Gissing sucked in the home cover and wing Shane Horgan was put away unopposed.
Llanelli continued to press, however, and they enjoyed an eight-point cushion at the break when lively scrum-half Dwayne Peel darted through a gap to cross in the corner with Jones converting.
A 13-5 half-time lead quickly became 16-5 through a third Jones penalty and, although fly-half Felipe Contepomi pulled a penalty back for Leinster, the rest of the half belonged to the Scarlets.
First wing Garan Evans crossed in the 53rd minute following by full-back Barry Davies and man of the match Peel, who claimed his second and the crucial bonus point.
Canadian prop Jon Thiel claimed Llanelli’s fifth of the night, Davies the sixth and a late double from replacement John McWeeney was nothing but a consolation for Leinster.
: B. Davies, Evans., Taylor, Selley, Finau, S. Jones, Peel, I. Thomas, A. Gravelle, Thiel., A. Jones, Wyatt, Easterby, Boobyer, Hodges.
Llanelli Scarlets: Tries: B. Davies 2, Evans., Peel 2, Thiel.. Cons: S. Jones 6. Pens: S. Jones 3.
: Dempsey, Horgan, O’Driscoll, D’Arcy, Burke, Contepomi, O’Riordan, Corrigan, S. Byrne, Coyle, O’Kelly, Gissing, Costello, Ronan, Dillon.
Leinster: Tries: Horgan, McWeeney 2. Cons: Contepomi. Pens: Contepomi.