A hatrick of tries from Jamie Elliot and further tries from Manoa, Van Velze and Tonga’uiha were enough to see the hosts prevail.
Guy Armitage crossed for a late consolation for Irish seven minutes from time.
London Irish went into the break 14-9 down but were possibly the better side in the first half. Tom Homer kicked the Exiles into an early lead with a penalty on 8 minutes after great work by Jon Fisher who carried twice in the build-up to the penalty.
The Saints response was immediate when young winger Jamie Elliot dived over in the corner. Stephen Myler sent a wide pass to the winger who just had enough strength to power over. Myler’s conversion gave the Saints a 7-3 lead.
The remaining half an hour was evenly contested with the Saints forwards just shading the battle up front and the Exiles backs looking far more dominant that their Saints counterparts. Tom Homer added two more penalties to give Irish the lead moving toward half time.
However the hosts went into half time in front with a try from Samu Manoa on the stroke of half time.
Saints started the second half superbly when Soane Tonga’uhia powered his way over for a try. Myler converted to give the home side the biggest lead of the day at 21-9.
Saints dominance continued when first GJ Van Velze crossed for the bonus point try and then Jamie Elliot sprinted away to finish off a great backline move.
Irish did get one back with replacement Setaimata Sa showcasing his passing abilities in the lead up with a great pass to Guy Armitage who finished well in the corner.
The Exiles welcome Worcester Warriors to Madejski Stadium on 23rd March for the St Patrick’s Party.
Score sequence (London Irish first): 3-0, 3-7, 6-7, 9-7, 9-14, 9-21, 9-28, 9- 33, 14-33, 14-40
Scorers: London Irish: Penalties: Homer (8, 34, 38); Tries: Armitage (73); Northampton Saints: Tries: Elliot (9, 58, 76), Manoa (40), Tonga’uiha (44), Van Velze (54), Elliot; Conversion: Myler (4), Lamb
15. Ben Foden; 14. James Wilson; 13. George Pisi; 12. Tom May 9rep: Dom Waldouck, 54); 11. Jamie Elliot; 10. Stephen Myler (rep: Ryan Lamb, 53); 9. Lee Dickson (rep: Martin Roberts, 53); 1. Soane Tona’uiha (rep: Alex Waller, 59); 2. Dylan Hartley (rep: Ross McMillan, 68); 3. Tom Mercey (rep: Brian Mujati, 51); 4. Courtney Lawes; 5. Christian Day; 6. Samu Manoa (rep: Mark Sorenson, 59); 7. Phil Dowson (rep: Ben Nutley, 27-34, 51); 8. GJ Van Velze
Replacements: 16. Ross McMillan; 17. Alex Waller; 18. Brian Mujati; 19. Mark Sorenson; 20. Ben Nutley; 21. Martin Roberts; 22. Ryan Lamb; 23. Dom Waldouck
15. Tom Homer; 14. Topsy Ojo; 13. Guy Armitage; 12. Shane Geraghty (rep: Setaimata Sa, 69); 11. Marland Yarde; 10. Ian Humphreys; 9. Pat Phibbs 9rep: Jack Moates, 59); 1. Max Lahiff (rep: Jerry Yanuyanutawa, 73); 2. Scott Lawson (rep: David Paice, 51); 3. Halani Aulika (rep: Leo Halavatau, 71); 4. George Skivington (rep: Kieran Low, 73); 5. Bryn Evans (Captain); 6. Matt Garvey (rep: Jebb Sinclair, 51); 7. Ofisa Treviranus; 8. Jon Fisher (rep: Jamie Gibson, 69)
Replacements: 16. David Paice; 17. Jerry Yanuyanutawa; 18. Leo Halavatau; 19. Kieran Low; 20. Jebb Sinclair; 21. Jamie Gibson; 22. Setaimata Sa; 23. Jack Moates
Referee: JP Doyle
Attendance: 12, 465
Show your "Exile-Style" and wear it with pride!