Richards will leave Gloucester for his second spell with the Exiles after beginning his career at Sunbury in 1996.
The 29-year-old has six England caps and was a member of the team that won the 2005 Hong Kong Sevens.
Irish director of rugby Brian Smith said: "Peter is an important signing for us.
"With up to 35 top class games to play each season we need two scrum-halves of real, international calibre if we are to be competitive.
"It is asking a lot of one player to play to a high standard in so many games.
"We want to be in a position where all our backs are capable of scoring tries. Peter has a proven track record in that regard."