Leo Cullen’s men were made to pay for a sloppy start, allowing the Scarlets to build a big lead, and a long second half lay head after winger Barry Daly’s 41st-minute red card for tackling Aled Thomas in the air.
However, the visitors really knuckled down after that with a number of young players stepping up to the plate. Although they leaked further tries to Will Boyde and Ryan Elias, going 35-8 down, Leinster outscored the Scarlets by three converted tries to a lone penalty in the closing stages.
Richardt Strauss’ 39th-minute effort, which involved some lovely running from the hooker himself and Noel Reid and Adam Byrne, was added to by seven-pointers from young guns Byrne (55 minutes), Tom Daly (59) and Andrew Porter (80).
Head coach Cullen agreed that there were ‘some positives’ to take forward into next week’s home clash with the Dragons, and Leinster’s list of debutants for the season was added to by the introduction of Terenure College prop Oisin Heffernan for the final 15 minutes.
Lying top of the table but without the vast majority of their Ireland internationals, Leinster were second best in a rip-roaring first quarter at Parc y Scarlets. New Zealander Johnny McNicholl had a debut try for the Scarlets in the seventh minute, collecting an unfortunate fumble by Strauss on the Leinster 10-metre line to score to the right of the posts.
Rhys Patchell converted and Strauss almost made amends a few minutes later, Dan Leavy and captain Isa Nacewa going close to putting the lively front rower away with the home defence under increasing pressure.
While the excellent Ian Nagle led some robust defending from the Leinster pack, his second row partner Mike McCarthy paid the price for successive team penalties. He was sin-binned in the 21st minute and from the resulting kick to the corner, tighthead Werner Kruger was driven over for a second converted try.
Wingers Barry Daly and Byrne were every willing runners for the 14 men, a 26th minute penalty from Nacewa comfortably slotted through the uprights for the province’s first points of the night.
But, just before McCarthy’s return to the pitch, the Scarlets got up close to the try-line off multiple phases and then winger Steff Evans muscled his way over to the left of the posts with out-half Patchell again converting.
Evans would have had a quick-fire brace before the break but he dropped the ball at the crucial moment, and Leinster duly reduced the arrears to 21-8 for half-time, Dominic Ryan providing the initial spark before Byrne did really well to cut in off the right wing and link with the supporting Strauss for a well-worked try. Nacewa’s conversion attempt went wide.
Barely a minute into the second period, Leinster’s hopes of drawing closer on the scoreboard took a huge hit. After consulting with TMO Neil Hennessy, referee Sean Gallagher reached for his red card to dismiss Barry Daly for taking Thomas out in the air, with the Scarlets full-back landing heavily on the turf.
The Llanelli outfit’s slick handling opened up the Leinster defence out wide in the 42nd minute, Evans and Thomas combining to put number 8 and man-of-the-match Boyde over in the corner for the bonus point score, converted with aplomb by Patchell.
Cullen dipped into his reserves during a scrappy third quarter, Leinster coping better with their numerical disadvantage until the hosts turned possession from a close-in lineout into a try from hooker Elias, who rumbled over with support from Aaron Shingler.
Then came Leinster’s spirited fightback. Just three minutes after Elias’ converted effort, winger Byrne scored his fifth PRO12 try of the season, finishing smartly after a powerful Nagle-inspired lineout drive. Ross Byrne provided the scoring pass in midfield, aided by replacement Tom Daly’s decoy run.
Byrne and Daly were also to the fore for Leinster’s third try, the increasingly influential Daly darting in under the posts to ground the out-half’s brilliantly-weighted grubber kick. Nacewa quickly converted for a 35-22 scoreline.
The Scarlets broke up Leinster’s run of scores with a 71st minute penalty from replacement Dan Jones. That was enough to get them over the finish line, although the visitors’ persistence was rewarded in the dying seconds when replacement prop Porter picked from a close-in ruck and showed great strength to burrow over.
The bonus point for four hard-earned tries in west Wales keeps Leinster temporarily top of the table, just ahead of the Ospreys with third-placed Munster still to play Benetton Treviso tomorrow.
SCARLETS SCORERS: J. McNicholl (1 try), W. Kruger (1 try), S. Evans (1 try), W. Boyde (1 try), R. Elias (1 try), R. Patchell (5 conversions), D. Jones (1 penalty)
LEINSTER SCORERS: R. Strauss (1 try), A. Byrne (1 try), T. Daly (1 try), A. Porter (1 try), I. Nacewa (3 conversions and 1 penalty)
SCARLETS: Aled Thomas; Johnny McNicholl, Steff Hughes, Hadleigh Parkes CAPTAIN, Steff Evans; Rhys Patchell, Jonathan Evans; Wyn Jones, Ryan Elias, Werner Kruger, Tom Price, David Bulbring, Aaron Shingler, James Davies, Will Boyde.
REPLACEMENTS: Emyr Phillips, Dylan Evans, Peter Edwards, Tadhg Beirne, Josh Macleod, Aled Davies, Dan Jones, Gareth Owen.
15. Isa Nacewa CAPTAIN
14. Adam Byrne
13. Rory O’Loughlin (Tom Daly, 53)
12. Noel Reid
11. Barry Daly
10. Ross Byrne
9. Jamison Gibson Park (Luke McGrath, 44)
1. Peter Dooley (Andrew Porter, 53)
2. Richardt Strauss (James Tracy, 44)
3. Michael Bent (Oisin Heffernan, 64)
4. Mike McCarthy (Ross Molony, 53)
5. Ian Nagle (Peadar Timmins, 74)
6. Dominic Ryan (Zane Kirchner, 44)
7. Dan Leavy (Peadar Timmins, 51-58, blood sub)
8. Jack Conan
REFEREE: Sean Gallagher (IRFU); ASSISTANT REFEREES: Elgan Williams (WRU), Leo Colgan (IRFU); TMO: Neil Hennessy (WRU).