Speaking about the move, the 30-year-old said: "I'm very excited to be joining London Irish. They play an attractive style of rugby that I am looking forward to being involved in," said Humphreys.
Irish director of rugby Brian Smith added: "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, now we have a pair of Irish half backs as good as any in Europe joining our current talent."