Leinster thrashed Ospreys 36-3 on Friday to book their place in rugby's European 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 after 20 minutes.
Although the Welsh region initially held out bravely, they soon crumbled as Leinster made the extra man count.
Two penalty tries and touchdowns from Cian Healey, Jordi Murphy and Isaac Boss ensured Leinster took the bonus point to preserved their slim chances of securing a home quarter-final, although that would depend on a couple of group winners slipping up this weekend.
The three-time winners became the seventh team to secure a last-eight berth with just one runners-up wildcard 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."
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 knockout stages, as long as Saracens lose at home to Connacht on Saturday.
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