The red-hot Scarlets raced into a 21-8 half-time lead, thanks to tries from Jonny McNicholl, Werner Kruger, and Steff Evans, with Richard Strauss crossing in reply.
However, things went from bad to worse for Leo Cullen when Barry Daly’s dangerous hit on an airborne Scarlets full-back Aled Thomas earned him a red-card, and allowed the hosts to go further ahead through Will Boyde and Ryan Elias.
However, the 14-men rallied to get a bonus-point, thanks to Adam Byrne and Tom Daly, before Andrew Porter crashed over at the death.
Leinster found themselves playing catch-up when hooker Strauss popped up out-wide but fumbled the ball into the grateful hands of Scarlets debutant Jonny McNicholl, who ran-in unopposed for a dream start to his Pro12 career.
Second-row Mike McCarthy was shown yellow after 22 minutes and the league leaders were made to pay, as the Scarlets shunted tighthead prop Werner Kruger over the whitewash from the resulting lineout.
The game looked to have been won before half-time when Evans crashed over from close-range, but Strauss handed Leinster a lifeline. Noel Reid broke downfield and found Adam Byrne in support, and the flying wing offloaded to Strauss, who dived over to make it 21-8 at half-time.
The nightmare became a reality for Cullen less than a minute after the restart when Daly was shown the red card.
Scarlets bagged the bonus-point moments later when Evans sliced through and found Thomas, who put No 8 Boyde over at the corner, before Elias put the gloss on the victory.
However, 14-men Leinster finally rallied from 35-8 down to get themselves back in the hunt for a losing bonus-point.
Firstly, Byrne sliced through to score at the posts, before Ross Byrne found Daly with a delicate grubber to get Leinster back to within 13 points.
Cullen’s men showed their character to grab a try bonus-point thanks to Porter at the death.
Tries: McNicholl, Kruger, S. Evans, Boyle, Elias. Cons: Patchell 5. Pens: D. Jones.
A. Thomas, McNicholl, S. Hughes, Parkes, S. Evans, Patchell, J. Evans, W. Jones, Elias, Kruger, Price, Bulbring, Shingler, J. Davies, Boyle.
Owen for A. Thomas (55), D. Jones for Patchell (60), A. Davies for J. Evans (55), D. Evans for W. Jones (54), E. Phillips for Elias (54), Edwards for Kruger (72), Beirne for Price (50), Macleod for Bulbring (54).
Tries: Strauss, A. Byrne, T. Daly, Porter. Cons: Nacewa 3. Pens: Nacewa.
Nacewa, A. Byrne, O’Loughlin, Reid, B. Daly, R. Byrne, Gibson-Park, Dooley, Strauss, Bent, M. McCarthy, Nagle, D. Ryan, Leavy, Conan.
T. Daly for O’Loughlin (54), L. McGrath for Gibson-Park (45), Porter for Dooley (54), Tracy for Strauss (45), Heffernan for Bent (62), Molony for M. McCarthy (54), Kirchner for D. Ryan (45), Timmins for Leavy (52).
Shaun Gallagher (RFU).
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