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.
He has been capped three times by New Zealand in rugby league and five times in union by Samoa as a centre.
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."
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