Northampton 27 (10)
A disconsolate Ben Cohen leaves the field after relegation is confirmed
Tries: Reihana, Lamont, Robinson
Cons: Reihana 2
Pens: Reihana 2
London Irish 22 (12)
Tries: Ojo, Mapasua 2
Cons: Mordt, Bishop
Northampton's last-day victory over the Exiles was all in vain as Worcester beat Saracens to condemn the Saints to relegation to National League One.
The hosts did everything they could, recovering from an early Topsy Ojo try and a 12-10 half-time deficit.
Bruce Reihana and Sean Lamont crossed for tries either side of the interval, Seilala Mapusua going over for Irish.
Mark Robinson's try took Saints clear, Reihana converting all three tries in a 17-point haul, but it was to no avail.
The Exiles had the last word with Mapusua scoring his second try, but the result was immaterial to the visitors, having secured sixth place in the Premiership.
Saints, by contrast, knew their fate could be decided elsewhere, and that was how it proved as they agonisingly finished a point adrift of Worcester, who beat Saracens 22-7.
It is the second time Northampton have had to face up to relegation, having also dropped out of the top flight in 1995 before returning under Ian McGeechan's stewardship.
Chairman Keith Barwell has already promised a major review, opening his programme notes with an apology to fans for the club's predicament.
Ultimately you get what you deserve and we deserved what we got
Saints coach Paul Grayson
"We cannot continue the way we are," he wrote. "We crave success on the park and we need a coaching structure and a playing culture that can provide that."
That point was illustrated by the fact that this was Saints' first victory on home soil in 2007.
They belied their lowly status with wins at Leicester, and Biarritz in the Heineken Cup, but too often failed to find the same inspiration for more mundane league contests.
Shaking off the fatigue of their bruising Heineken Cup semi-final defeat to Wasps six days earlier, they overcame a lively Exiles outfit for whom Justin Bishop, Riki Flutey, Neal Hatley, Olivier Magne and Richie Rees were playing their last games.
Flutey sent Ojo weaving through some shoddy Saints defence early on, but the hosts' Kiwi contingent came up trumps when Robinson sent Carlos Spencer racing through before the fly-half's looping pass sent Reihana over.
Reihana converted and added a penalty before Irish hit back with a lovely try by Mapusua to lead 12-10 at the interval.
Saints rallied on the resumption and after the pack battered away from close range, Lamont barged his way over, Reihana again converting.
The full-back added his second penalty before Robinson scampered over for Saints' third try.
But with news filtering through from Sixways of Worcester's victory, the final whistle signalled only the grim reality of relegation.
London Irish coach Brian Smith:
"We are disappointed we did not win the game but that is insignificant when you understand what happened to Northampton today. Saints are a good side.
"They've obviously had their problems this season but the two teams in the drop-zone have won well at home today which speaks volumes for the standard of the league."
Northampton: Reihana, Lamont, Clarke, Kydd, Cohen, Spencer, Robinson, Smith, Cortese, Brooks, D. Browne, Short, Tupai, Lewitt, Labirt. Replacements: Richmond, Budgen, Gerard, Fox, J. Howard, Myler, Diggin.
London Irish: Ojo, Bishop, Mapusua, Mordt, Tagicakibau, Flutey, Hodgson, Hatley, Paice, Lea'aetoa, Kennedy, Casey, Roche, Danaher, P. Murphy. Replacements: Collins, Coetzee, J. Hudson, Johnson, Magne, Rees, D. Armitage.
Referee: Chris White (RFU)