Saints were forced to play the final 23 minutes at Franklin’s Garden without Dylan Hartley who was sent off for a swinging arm challenge on Sean O’Brien, one of Leinster’s five try scorers.
The visitors took a tighter grip of top spot in Champions Cup Pool 4 with closing touchdowns from replacements Rory O’Loughlin and Jamison Gibson Park, allied to captain Isa Nacewa’s 78th-minute effort which came after George Pisi’s sin-binning.
It was an attritional night for both sides, Leinster losing Joey Carbery to an ankle injury but an early scoring blitz, which included Garry Ringrose’s first European try, gave them a 10-3 half-time lead.
Leo Cullen’s men looked like they were in for a real scrap after Rob Kearney limped off and Northampton replacement Ahsee Tuala finished off good work by Kerry man JJ Hanrahan to bring the hosts level.
However, Nacewa calmed things down with a tremendous 47-metre penalty goal and the dismissal of England captain Dylan Hartley, who had only been on the pitch for six minutes, helped Leinster on their way to a comfortable victory in the end.
The result leaves the province six points clear ahead of Sunday’s French derby between Montpellier and Castres, with fourth-placed Northampton making the return trip to Dublin on Saturday week (December 17).
The visitors were ahead inside two minutes, Kearney’s clever dummy and fend in midfield opening up the Saints defence, and his pass in the 22 released Ringrose to finish off and Nacewa quickly converted.
Full-back Kearney looked very sharp in the opening exchanges, almost wriggling free over halfway from a kick return, and the game’s second scrum yielded a central penalty which Nacewa slotted over for 10-0.
Leinster lost the fast-starting Carbery to an ankle injury after just a quarter of an hour, meaning a Champions Cup debut for fellow 21-year-old Ross Byrne at out-half. Saints, who began to settle, forced a penalty at the breakdown which Stephen Myler turned into three points.
Led by talismanic flanker Tom Wood, Northampton grew more into the game after Leinster failed to profit from Luke McGrath’s snappy break up to five metres out. The hosts replied with some some threatening phases in the 22.
The Leinster attack came alive again entering the final 10 minutes of the first half. Good distribution by Ross Byrne and centres Robbie Henshaw and Ringrose handed European debutant Adam Byrne an opportunity to stretch his legs up the right wing.
A couple of phases later, Josh van der Flier was bundled into touch just over five metres out. Two lineout steals robbed Leinster of decent positions and they failed to take advantage of a forwards-led onslaught approaching the interval. Credit to the home defence for some superb tackling.
O’Brien had an opportunist try – straight from his own kick ahead – ruled out for a knock-on of a ricocheting ball, the previous phase seeing winger Byrne gobble up a high ball and brilliantly use his strength to get past three defenders near halfway.
Leinster’s inability to convert that pressure into points came back to bite them five minutes into the second half. An error from Luke McGrath at a maul handed possession back to Saints and Hanrahan duly conjured up a try, making an initial break and then looping an inviting pass out for winger Tuala to finish in the right corner.
A smashing conversion from Myler was cancelled out by Nacewa, who had the accuracy and distance from a long range penalty on the left, and Leinster’s all-international front row – including man-of-the-match Tadhg Furlong – earned their corn by winning two successive scrum penalties following Hartley’s introduction off the bench.
That hard work paid off when Ross Byrne plunged a penalty towards the corner, the maul was set and when possession was moved closer infield, O’Brien – with timely support from back row colleague Jamie Heaslip – managed to muscle his way over with Nacewa converting for 20-10.
Two minutes later, Hartley’s reckless challenge to the head of O’Brien led to referee Jerome Garces producing his red card having reviewed the footage with TMO Eric Gauzins.
Both coaches sent on reinforcements heading into the final quarter, Leinster’s control of the scrum continuing as 14-man Saints were shoved back and van der Flier dribbled through twice to heap pressure on the home defence. From the resulting lineout, Ross Byrne’s pinpoint cross-field kick out to the left was collected by the leaping O’Loughlin for a well-worked 65th-minute try that went unconverted.
O’Brien had been unable to continue and out-half Byrne was also in the wars during the final quarter, his head needing to be bandaged up after a clash of heads with try scorer O’Loughlin drew blood.
Ringrose and Henshaw both put boot to ball out wide as Leinster hunted for an elusive bonus point score. It finally arrived in the 74th minute, Heaslip stripping possession from Saints on halfway and replacement scrum half Gibson Park was fed on the left wing, the New Zealander scampering into space and his dancing feet saw him cut inside three tired defenders for a sparkling solo try, converted by Nacewa.
Northampton ended the game down to 13 players after replacement centre Pisi saw yellow for a high tackle on Nacewa as he stepped inside him. On a night when Devin Toner made his 50th European Cup appearance, Leinster boosted their scoring difference further when Nacewa, who was not held in a tackle by Tuala, did really well to reach over near the left corner.