Northampton secured second place in the Premiership and a home play-off semi-final against Saracens with a comfortable win over London Irish.
Saints were 11-0 up at the break after the first of Jon Clarke's two tries and two Stephen Myler penalties.
Chris Ashton then scored his 23rd try of the season while Clarke added another late on.
It was a disappointing way for World Cup winner Mike Catt to end his career in domestic rugby.
Catt, 38, came off the bench for the last 13 minutes for a cameo appearance for Irish, for whom he will continue to be attack coach.
The hosts' only points came from Danie Coetzee's 65th-minute try, converted by Tom Homer, as Toby Booth's men ended the season in sixth place.
Although defences dominated early on, Myler kicked two penalties before Northampton pounced for a try just before the break with Ashton sending Clarke down the left wing for the centre to scamper over for his eighth of the season.
Although Myler missed the conversion and then a penalty chance early in the second half, Saints extended their advantage when Ashton went over in the 48th minute under the post and this time Myler was on target.
The fly-half kicked his third successful penalty of the afternoon for 21-0 as the home side struggled.
Coetzee's close-range try boosted their spirits but Northampton's joy at victory was tempered by an injury to full-back Ben Foden who was helped from the pitch and immediately given treatment by the Saints medical staff for a sprained ankle
Northampton director of rugby Jim Mallinder:
"Tremendous for Chris (Ashton) to come back. He has had a really difficult week and are thoughts are with his family. We're really delighted that he came back and played for us.
"We wanted a professional performance and for large parts of the game, we dominated.
"We were very solid, got some good turnovers, the scrum was good, we were fairly patient, played in the right areas of the field and we know we are a side who can score tries."
"We've had some close games against Saracens this season. They are playing well - and played very well when they came to our place. That hurt, hearing them singing as if they'd won the league.
"It'll be an almighty battle but we think we have a side to win that game."
London Irish head coach Toby Booth
"I'm very disappointed. Sometimes you have to hold your hand up and say you were beaten by a better side."
London Irish: Homer, Ojo, Joseph, Seveali'i, Tagicakibau, Lamb, Hodgson, Corbisiero, Coetzee, Rautenbach, Roche, Garvey, Thorpe, S. Armitage, Hala'ufia. Replacements: Catt for Ojo (66), Rudd for Tagicakibau (48), Lennard for Lamb (58), Murphy for Corbisiero (58), Herring for Coetzee (70), Tideswell for Rautenbach (66), Perry for Garvey (58), Gibson for Hala'ufia (41).
Northampton: Foden, Ashton, Clarke, Downey, Reihana, Myler, Dickson, Tonga'uiha, Hartley, Murray, Fernandez Lobbe, Kruger, Dowson, Best, Wilson. Replacements: Geraghty for Foden (66), Dreyer for Tonga'uiha (76), Sharman for Hartley (76), Mujati for Murray (52), Lawes for Fernandez Lobbe (56), Easter for Dowson (76). Not Used: Dickens, Ansbro.
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