Scarlets (10) 17
Tries: Roberts, Manu Cons: Priestland 2 Pen: Priestland
Leinster (23) 31
Tries: Nacewa, Jowitt, Jennings Cons: Contepomi 2
Pens: Contepomi 4
Scrum-half Martin Roberts breaks away for his first-half try
Leinster maintained their challenge for the Magners League title and put and dent in the Scarlets' hopes with a well-earned win at Parc y Scarlets.
The visitors led 23-10 at the break and withstood a determined home fight-back in the second-half.
Isa Nacewa, Cameron Jowitt and skipper Shane Jennings scored Leinster tries and Felipe Contepomi kicked 16 points.
Scarlets tries came from Martin Roberts and Deacon Manu while Rhys Priestland struck seven points.
Contepomi struck his first penalty in the second minute and 19th minutes while between those kicks opposite number Priestland was unable to respond with his first shot at goal.
The hosts then had to endure the sight of star Kiwi centre Regan King being carried off to be replaced by Gavin Evans and more misery was to follow.
Nacewa raced away unopposed following an excellent Jowitt off-load and the Argentine fly-half converted.
Jowitt then profited from Girvan Dempsey's charge-down of a Priestland clearance and Contepomi again converted to give his side a 20-0 lead after 26 minutes.
Priestland stuck back in thee 28th and the hosts finally received some reward for their attacking intent as scrum-half Roberts glided past prop Stan Wright to score, his half-back partner converting.
However, Contepomi struck his fourth shot from as many attempts in the 35th minute to give his side a 23-10 lead at the break.
Priestland's poor night with the boot continued after the break, his misses in the 43rd and 45th minutes.
At the next opportunity, skipper Simon Easterby ordered the young fly-half to kick for touch and with attack through the hands their only option the hosts began to claim the initiative.
Scrum-half Roberts was central to their efforts, prompting and probing expertly amid some dynamic phase play that ended with prop Manu barging over after 53 minutes.
Priestland converted to put his side within a converted try of taking the lead, but the Scarlets were soon facing another fight-back scenario.
Replacement Jonathan Sexton's one-two with Shane Horgan broke the home defence and skipper Jennings burst clear.
Contepomi was unable to convert that 62nd-minute try, but made amends a minute later with a penalty only to be sin-binned six minutes later for failing to release after tackling Phil John at the end of the prop's storming run.
The Scarlets continued to attack, but were unable to take advantage of Contepomi's absence.
Leinster boss Michael Cheika:
"We were starting to think we couldn't score one [a try] again because we'd gone for a while without one, everyone was saying.
"But we knew that if we just concentrated on our mental approach and how hungry you want to be to win the game, but also to maintain the technique and the style, the shape of the team [we'd be okay].
"Unfortunately we couldn't get the fourth one, which would have been nice because we haven't got a lot of bonus points this year - no-one seems to have - but I won't be greedy. I'll be happy to come down here to win."
Scarlets: Stoddart; Daniel, King, Davies, M Jacob; Priestland, Roberts; I Thomas, Owen, Manu, Reed, Day, Cooper, Easterby (capt), Lyons.
Replacements: G. Evans for King (19), John for I Thomas (57), Schwalger for Owen (58), Turnbull for Day (49).
Not Used: McCusker, Cattle, C. Thomas.
Leinster: Dempsey; Horgan, McFadden, D'Arcy, Nacewa; Contepomi, O'Donohoe; Healy, Blaney, Wright, Hogan, O'Kelly, Jowitt, Jennings, Elsom.
Replacements: Sexton for McFadden (51), Simon Keogh for O'Donohoe (59), Jackman for Blaney (41), McCormack for Wright (74), Toner for Hogan (60), Brown for Jowitt (73), Stephen Keogh for Jennings (80).
Ref: Andy McPherson (SRFU).