Now, provided Stade Francais beat Begles Bordeaux today, the previously no-hopers from Wales will win through to the quarter-finals.
Llanelli, surprise conquerors of Stade Francais at Stradey Park last Saturday, needed to reproduce similar passion and commitment to upset Leinster. They didn't.
Llanelli found themselves a try adrift in the seventh minute and then 21 points behind inside 20 minutes as Leinster, playing with fury and confidence, plundered three tries, all converted by Alan McGowan.
Two of the tries were scored by flanker Declan O'Brien, who arrived at the ground expecting to sit on the bench but took his chance with both hands when Brennan was declared unfit.
The first try came with Leinster winning a line-out and the pack driving O'Brien over the line. Llanelli had barely recovered from that when Ciaran Scally took a quick penalty to breach Llanelli's defence for McGowan to finish off the move with a neat sidestep inside his opposite number, Stephen Jones.
In the 18th minute Leinster struck again when Colin McEntee picked up and drove, the ball was recycled for Kevin Nolan to maintain the momentum and finally for super-replacement O'Brien to score his second.
But whatever Llanelli may have felt at these early shocks they were quick to respond, producing a stunning move with the ball moving through 10 pairs of hands before Wayne Proctor scored wide out.
Ian Boobyer scored a second just before half-time, which Stephen Jones converted, and it was Leinster who started to wilt as Neil Boobyer drove over for Llanelli's third try in injury time at the end of an astonishing first half.
Leinster: Tries O'Brien 2, McGowan; Conversions McGowan 3; Penalties McGowan 2. Llanelli: Tries Proctor 2, Ian Boobyer, Neil Boobyer, Wyatt, Roddy Boobyer; Conversions Stephen Jones 2.
Leinster: C Clarke; K Nowlan, S Horgan, K McQuilkin (B Carey, 69), G Dempsey; A McGowan, C Scally; R Corrigan, S Byrne (P Smyth, 72), E Byrne (H Hurley, 67), H Kos (P Holden, 67), G Fulcher (capt), D O'Brien, P Ward, C McEntee.
Llanelli: D Williams; W Proctor, N Boobyer, R Boobyer, G Evans; S Jones, R Moon; P Booth, R McBryde (capt), M Madden, V Cooper (M Morgan, 40), M Voyle, D Hodges, I Boobyer (I Jones, 77), C Wyatt. Referee: J Dume (France).Reuse content