Humphreys, who played for Leicester before joining Ulster in 2008, has agreed a three-year deal with the Exiles.
He will join Irish in time for pre-season training, although his immediate priority is the May 19 Heineken final between Ulster and Leinster at Twickenham.
Humphreys has represented Ireland at under-19, under-21 and A levels, and also captained his country in the 2005 Rugby World Cup sevens tournament.
Irish rugby director Brian Smith said: "It is fantastic to have Ian on board for the next three seasons. He will bring with him a wealth of experience and leadership qualities.
"He is a constant threat with ball in hand and has a great ability to create space for his outside backs. He also has a fantastic kicking game.
"When I was previously at the club we tried to recruit Ian and Tomas O'Leary, and now we have a pair of Irish half-backs as good as any in Europe."
Humphreys, 30, and O'Leary are among a clutch of new signings for next term that also includes Catalan Dragons rugby league star Setamaita Sa, Leicester lock George Skivington and Gloucester hooker Scott Lawson.
Humphreys added: "I am very excited to be joining London Irish.
"They play an attractive style of rugby that I am looking forward to being involved in. My immediate focus, however, is with Ulster as we prepare for the Heineken Cup final."