And for Connacht, too, who swept to the top of the table as a result.
The three-time European champions had misplaced their ability to eke out this type of win last season under Matt O’Connor, but they recovered from a slow start to boss this game the longer it went on and it will stand to them as they turn their attention to Europe next week.
It wasn’t all good news.
The sight of Rob Kearney going off after 24 minutes with a hamstring injury wasn’t ideal ahead of the first round of Champions Cup fixtures, although Leinster had a waterboy last night who could probably do a decent job there.
His name? Isa Nacewa.
This was always likely to be an intriguing clash between a home team inhabiting a Pro12 play-off space for the first time in 12 months — digest that just for a minute — and a visiting selection riding high at the league summit on the back of six victories from six so far this season.
The visitors played with an aggression at the breakdown that should have been dealt with more firmly early on by referee Leighton Hodges who somehow limited his punishment to a yellow card for Jake Ball after eight minutes, and that for pulling down a maul.
Yet, the visitors were entitled to their own moans about the officiating and, though their red, orange and yellow kit looked like something a child would dream up in kindergarten, then some of their early play was far more pleasing on the eye.
Their first try took just four minutes to construct with centre Hadleigh Parkes going under the posts. It should never have come to that as Hodges harshly penalised Dominic Ryan for a light brush at loosehead Rob Evans’ face just five metres from the Leinster line.
Within moments, the five points was conceded.
There was no quibbling with their second, a well constructed effort that arose from scrum-half Gareth Davies’ half-break and a pair of subsequent offloads which allowed Gareth Owen to sneak in at the corner. A Dan Jones penalty completed their first-half scoring.
Leinster had shown glimpses of encouraging play. Sean O’Brien was lively, James Tracy busy as a bee on his first start at hooker and Cian Kelleher full of threat on the wing, but their rhythm was intermittent.
Hardly surprising, in fairness, given Cullen made 15 changes this week as he sought to reintegrate the last of the returning World Cup squad members.
Ian Madigan’s three penalties were the sum of the province’s response in the first 40 minutes although it took just five more minutes for Mike Ross to claim their first try — his second in 121 games — with the last of a succession of pick-and-goes which Scarlets simply couldn’t defend.
Madigan’s conversion gave Leinster the lead for the first time, 16-15, and he added another penalty on the hour, by which time Jonathan Sexton had appeared off the bench for his first run-out at the RDS since May of 2013.
A ‘welcome back’ try was denied the Ireland out-half after 72 minutes thanks to some not-so-subtle obstruction from Noel Reid, but Cullen’s side held off a belated assault in the dying moments to claim the win ahead of the visit of Wasps to the RDS tomorrow week.
R Kearney; Z Kirchner, G Ringrose, N Reid, C Kelleher; I Madigan, I Boss; C Healy, J Tracy, M Ross; D Toner, T Denton; D Ryan, S O’Brien, J Heaslip. Replacements: B Te’o for Kearney (24); J McGrath for Healy and M Moore for Ross (both 51); J Sexton for Te’o (53); H Triggs for Denton (61); B Byrne for Tracy (66); D Leavy for Ryan (76).
A Thomas; T Williams, G Owen, H Parkes, M Tagicakibau; D Jones, G Davies; R Evans, E Phillips, P Edwards; J Ball, L Rawlins; A Shingler, J Davies, J Barclay. Replacements: H Robinson for Tagicakibau (55); M Collins for Thomas (60); M Paulino for Rawlins, K Owens for Philips and T Price for Ball (all 61); D Evans for R Evans (72); A Davies for G Davies (76).
L Hodges (WRU).