Roche, who has been on trial with the club, came into the picture after veteran lock Ryan Strudwick suffered an injury to his ankle ligament that has caused him to be out of action for at least three months.
Irish, who are currently third from the bottom of the Zurich Premiership and trying to stay clear of relegation, will be hoping that they can draw on Roche's experience. Roche, 23, previously played for Division One club, Rugby, and Zurich Premiership club, Saracens, and was in the Barbarians squad for its end-of-season tour in 2002.
Managing Director of London Irish, Conor O'Shea, was very pleased with the club's new signing stating: "We needed to augment our second row resources following Ryan's injury. We are pleased to have been able to recruit a player of Kieran's ability and experience."