Paris - Leinster thrashed Ospreys 36-3 on Friday to book their place in the Heineken Cup quarter-finals while Northampton kept their faint hopes alive with a 13-3 win over Castres.
Leinster looked like they were in for a battle royale with their Celtic League rivals until Welsh lock Ian Evans was sent off for stamping on 20 minutes.
Although the Welsh region initially held out bravely, they soon crumbled as Leinster made the extra man count.
Two penalty tries and scores from Cian Healey, Jordi Murphy and Isaac Boss ensured Leinster took the bonus point that preserved their slim hopes of securing a home quarter-final tie, although that would depend on a couple of group winners slipping up this weekend.
The three-time winners became the seventh team to secure their quarter-final berth with just one runners-up wild card slot still to be decided.
On an evening of good news for the Irish, South African-born hooker Richardt Strauss made his return to action just three months after undergoing an operation on a heart defect that was originally thought to have ended his season.
"It was pretty good, we had to stick to our guns," Healey told Sky Sports.
"It was a slow game, trying to piece things together was tough enough but we stuck to what Matty (coach Matt O'Connor) told us and pulled it off nicely at the end."
Flyhalf Jimmy Gopperth, who ended the game with 11 points, kicked a penalty on nine minutes before Dan Biggar equalised for the visitors four minutes later.
It looked like developing into a tight contest until Evans was caught stamping on Mike McCarthy's head in a driving maul.
McCarthy had to be taken off after the incident while the video umpire was summoned to help referee Romain Poite identify the culprit and send him off.
It took 13 minutes for Leinster to make the extra man count and when they did it was an unstoppable driving maul which Ospreys collapsed just short of their line, leading Poite to run under the posts and award the penalty try.
The Irish province almost had a second try after prop Martin Moore's initial barnstorming break before offloading to Gopperth.
The New Zealander got over the line as he was tackled by two Ospreys players but as he tried to ground the ball, a Welsh foot blocked him and forced the flyhalf to knock on.
It was a brilliant piece of defending but just three minutes later Leinster's pack pushed up to Ospreys line and Healey drove over from close range.
Leinster should have had a try 10 minutes into the second period but Dave Kearney knocked on barely a metre from the line.
The second penalty try came on 62 minutes as Leinster shoved Ospreys back over their own line from a five-metre scrum, only for scrumhalf Rhys Webb deliberately going offside to stop Jamie Heaslip touching down.
A few minutes later scrumhalf Eoin Reddan knocked on under pressure from Matthew Morgan as he went over the line looking for the bonus point try.
But the bonus came eight minutes from time as Spanish-born Murphy hit a perfect line to barge over.
The floodgates opened as Boss popped up on Luke Fitzgerald's shoulder to take a pass and run in, although Gopperth missed his first conversion attempt of the night.
Meanwhile, Northampton toiled throughout against the French champions but secured victory in the last minute with George Pisi's try after he charged down a Castres clearing kick inside their 22.
The win moved Saints onto 17 points which gives them a slight chance of progressing to the knock-out stages, as long as Saracens lose at home to Connacht on Saturday.
Heineken Cup pool results on Friday:
Leinster (IRL) 36 Ospreys (WAL) 3
Northampton (ENG) 13 Castres (FRA) 3
Saturday (all times SA)
Pool 2 (19:40)
Cardiff (WAL) v Exeter (ENG)
Glasgow (SCO) v Toulon (FRA)
Pool 3 (15:35)
Saracens (ENG) v Connacht (IRL)
Zebre (ITA) v Toulouse (FRA)
Pool 5 (20:00)
Leicester (ENG) v Ulster (IRL)
Montpellier (FRA) v Treviso (ITA)
Pool 4 (17:00)
Scarlets (WAL) v Harlequins (ENG)
Clermont (FRA) v Racing-Metro (FRA)
Pool 6 (14:45)
Munster (IRL) v Edinburgh (SCO)
Perpignan (FRA) v Gloucester (ENG)