He has been signed on a two-year contract from NEC Harlequins. Duffy, who was a team-mate of Deane's at 'Quins, is expected to play for Connacht against Leinster in their Magners League clash on March 24.
The 32-year-old Deane left Connacht seven seasons ago for Sale Sharks and life in the Premiership in England.
With his family ties in Lonford, the London-born midfielder spent a season with Connacht before following coach Glenn Ross to Sale.
He was capped by Ireland at 'A' and Sevens levels and played and scored a try for the senior team in a 56-3 win over a New Zealand Divisional XV in a tour match in 2002, partnering current Irish skipper Brian O'Driscoll in midfield.
Connacht CEO Gerry Kelly said that the return of Oâ00Connor, Duffy and now Deane, was a huge boost to the province.
"We are delighted to have the players back as we continue to strengthen our side. All of them are quality players and all are vastly experienced.
â00Invariably, in professional sport, there will always be comings and goings and it is great that three such talented players have decided to return," he admitted."
"We hope to be able to announce a number of other signings in the next few weeks as we continue to build a very strong side for next season."