Humphreys, 30, still has a year to run on his Ulster contract, but he will now join the Exiles this summer.
The former Leicester Tigers man, who failed to make the Ulster line-up for the Heineken Cup semi-final win over Edinburgh last weekend, has struggled for consistency this season.
Humphreys will be vying with Shane Geraghty - who arrives from Brive at the end of the campaign - for a place in London Irish's starting XV.
Meanwhile, Roche has confirmed he will be retiring from the game after 14 years as a professional.
Roche's final game for the club is this weekend's home clash against Gloucester and he is hoping to finish on a winning note.
"I would like to thank everyone who has played a part in my rugby career, from my school days onwards, including the players, coaches and support staff that I have worked alongside both at Saracens and London Irish," he told the club's official website.
"I'm looking forward to taking to the field one last time for London Irish at Madejski Stadium this weekend alongside some of those friends and hoping for a victorious send-off for everyone who is moving on."
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