Saints saw a strong start come to a shuddering halt as they were savaged 60-13 at the Aviva Stadium on Saturday night.
Jim Mallinder had made 10 changes to the side that was beaten 37-10 by Leinster at Franklin’s Gardens eight days earlier, and it was to be a soul-destroying night in Dublin.
Saints had gone 13-5 up after 21 minutes as a breakaway try from Ken Pisi and eight points from the boot of Stephen Myler made light of Adam Byrne’s early score for the hosts.
But there was to be no repeat of the heroics of 2013, when Saints won 18-9 in Dublin, as Leinster proceeded to eviscerate the team from Northampton.
The men in blue racked up a total of nine tries, with Saints chasing shadows during a gut-wrenching second half.
It was Saints’ biggest defeat since Mallinder took over in 2007, beating the 41-7 loss at Castres in October, and it was hard to watch for the Saints supporters who had shelled out to watch their team.
Saints had opted to leave out international stars such as Tom Wood and Louis Picamoles, and they well and truly paid the price on an horrific night for the club.
They will now have to regroup ahead of three vital Aviva Premiership games, with the pressure continuing to mount.
Saints were up against it from the off in Dublin as Nic Groom’s box kick was charged down by lofty lock Devin Toner, giving Leinster the chance to put the squeeze on.
And after some patient play from left to right, wing Adam Byrne scored in the corner.
Skipper Isa Nacewa missed the conversion and Saints soon had the chance to cut the gap, but Myler missed a penalty he would normally expect to make.
But he got the chance to make amends soon after as Saints built some pressure on the Leinster defence, forcing Sean O’Brien to offend at the breakdown.
Saints were bossing the breakdown battle and after Teimana Harrison won a penalty, Myler slotted it from in front of the posts to put his team ahead.
Leinster looked to respond, kicking to the corner, but Saints stood tall and stunned their hosts as Ken Pisi pouched the ball in his own 22 and sprinted away to score.
Referee Romain Poite looked set to disallow the try, but looked again and awarded it, meaning Myler could convert to earn an incredible eight-point lead after 22 minutes.
Nacewa immediately reduced the deficit with a well-struck penalty and after Saints left the door open soon after, scrum-half Luke McGrath sneaked in to score.
Nacewa converted and Leinster had their tails up, sensing that Saints were starting to crack a little.
And after George Pisi fumbled, Byrne was able to break from halfway to score his second try and Leinster’s third.
Nacewa again added the extras to earn a nine-point lead and Leinster still had time to rack up the bonus point before the break as flanker O’Brien found a way through.
Nacewa again rubbed salt in the wounds with the conversion and Leinster looked to have ended the game as a contest as they headed in at half-time with a 29-13 lead.
It was likely to be a damage limiation excercise for Saints, but they were to concede a fifth try just three minutes into the second period, as Tadhg Furlong powered over.
Nacewa converted and it promised to be a horrible final 35 minutes for Saints.
The floodgates had well and truly opened and Sean Cronin was the next to score, just four minutes later, with Ross Byrne neatly adding the extras.
Saints were soon down to 14 men as Jamie Gibson was sin-binned and Leinster quickly waltzed in for yet another try as Nacewa cut inside and beat some weak tackles to score.
The conversion was missed but Saints were 48-13 down and there were still 24 minutes of a chastening encounter to come.
Rory O’Loughlin was the next to get in on the scoring act for Leinster before Nacewa made it nine tries for the home side as a crossfield kick was spilled by Ken Pisi.
Saints thought they had scored before the end, but referee Poite and his TMO decided otherwise, and the game finally ended a few minutes later.
Leinster: Kirchner; A Byrne (O’Loughlin 51), Ringrose, Henshaw, Nacewa (c); R Byrne, L McGrath (Gibson-Park 58); J McGrath (Healy 48), Cronin (Tracy 48), Furlong (Bent 48); Toner (Ruddock 53), Triggs; O’Brien (Conan 57), van der Flier, Heaslip.
Saints: Tuala (Wilson 22); K Pisi, G Pisi (c) (Hutchinson 40), Hanrahan, Collins; Myler, Groom; E Waller (A Waller 67), Clare (Fish 63), Brookes (Hill 51); Craig (Paterson 51), Ratuniyarawa; Gibson, Clark (Nutley 40), Harrison.
Referee: Romain Poite (France)