The 28-year-old, who linked up with Jean-Pierre Elissalde's side last July, is joining the Exiles along with Wellington Hurricanes' fly-half Riki Flutey.
Willis, who scored a try for Beziers in this season's European Challenge Cup, is due to report for pre-season training next month at Irish's Madejski Stadium.
"Ben Willis is a talented scrum-half with a pass that is known for its length and accuracy. Both players will offer us more options in these crucial positions in the team."
Willis, who is also related to the great nineteenth-century English novelist Jane Austen, was released from his Leinster contract in May 2003.