Mapusua's deal will keep him at the club until 2011. He joined Irish last year.
Exiles rugby director Brian Smith said: "Seilala is a world-class player.
"He is as good as any centre playing the game, and we are delighted that he's committed himself to London Irish.
"Not only is he a brilliant rugby player; he's also an exceptional person who brings out the best in other players, both on and off the pitch."