Sa, 24, started his career playing rugby union before joinined National Rugby League side Sydney Roosters in 2006.
He left Sydney three years later after admitting charges of malicious damage, failing to leave a licensed premises and resisting arrest following an incident at a hotel.
He can play as as a line-breaking centre or a back-rower and Irish director of rugby Brian Smith believes he will be an explosive player.
"There's no doubt about it, Setaimata is an exciting and wonderfully skilful player and we're delighted that he will be joining the squad next season," he said.
"I have followed his NRL and Super League career very closely and he was coached at the Roosters by my good friend and current New South Wales State of Origin coach, Richy Stuart.
"We've needed to replace Seilala Mapusua in midfield for some time now and Sa fits the bill perfectly. He is an outstanding defender and a powerful runner with great physical and technical abilities.
"Still just 24 years old he already brings with him a wealth of international and top flight rugby experience, as well as winning a Rugby League world cup for New Zealand. We think he is going to take the Aviva Premiership by storm."
Sa has scored 10 tries in 39 games for Catalan Dragons and is a dual rugby league international, winning three caps for New Zealand.
On signing for Exiles, Sa said: "I'm really excited about joining London Irish. Returning to rugby union is something that I have been considering for a while, it has always been in my blood and testing my skills in the Aviva Premiership will be the ultimate challenge.
"The style of rugby that the Exiles play and the family values of the club suit me and my family well. I'm looking forward to getting down to it in the summer and I hope to be able to make an immediate positive impact."