The 25-year-old, who penned a three-year contact with the club last year, will now be available for the club indefinitely.
London Irish director of rugby Brian Smith told the club's official website: "It has been a long process but we are delighted that Sa can now join up with the squad.
"He is itching to be involved and the squad is looking forward to having him on board. Once he gets the hang of playing rugby union again there will be no better all-round midfielder in the country."
Sa has three New Zealand caps to his name.