The 24-year-old prop, capped 18 times by England, has made 76 first-team appearances for the Exiles but felt it was time for a new challenge after eight years at the club.
“I’ve really enjoyed my time at London Irish and am grateful for everything that the club has given me,” he said.
“I’m looking forward to what will be a new challenge at Saints next summer. There is a great culture and hopefully I can help the team succeed whilst
continuing my progression as a player, and I’m sure that Franklin’s Gardens will be a good environment for me in that regard.”
As well as snapping up the England loosehead prop, there was further good news for the Saints after they announced that Alex Waller and Tom Mercey have both agreed new deals.
Waller, 22, is the only player to have featured in every Saints match since the start of the 2011-12 campaign after playing in the 2011 Heineken Cup final and LV=Cup showpiece in 2012.
“It wasn’t a hard decision to make,” Waller said. “My family are in the area, I’ve been playing here for the last four or five years and I feel that I have developed a lot as a player. I want to keep developing and hope that I can do that under the guidance of the coaches and the set-up here at the Saints.
“I’m still learning every game and have played more than I thought I would, especially last season. Hopefully I’ve become an established member of the squad and can keep on pushing for a starting spot.”
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BIOGRAPHY: Marcus Rashford