It means he will link up again with his brother, Wales international flanker Aaron, who has agreed a new three-year contract with the Welsh region. Steven Shingler has also penned a three-year deal, having so far made 39 appearances for Irish.
"I have had some great experience from my time at London Irish, and it's been good for me to play in the intensity of the Premiership," he said.
"During my time at London Irish I've been lucky to work with and learn from some great people like Mike Catt, who have pushed me forward and improved my game.
"I am very grateful to have this opportunity to come back to Wales, and back to what I consider my rugby home, playing for a region I feel so strongly about."
Aaron Shingler, meanwhile, is part of the Wales squad currently preparing for this season's RBS 6 Nations Championship.
Scarlets head coach Simon Easterby said: "It's important for our progression to have secured these two talented players from our region who will add real value to our squad going forward."