He agreed a three-year contract with the Exiles in May last year, but has only just been given permission to join up with the squad from Super League side Catalan Dragons.
A schoolboy sensation in New Zealand, Sa was recruited by Australian rugby league side Sydney Roosters in 2006 before moving to the Dragons in 2010 where he scored 10 tries in 66 games.
Irish chief Brian Smith said: "It has been a long process but we a 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."
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.