Roly left Glasgow to return to South Africa this year to play in the Currie Cup for the Lions, who were knocked out in the semi-final.
"I am pleased and excited to be joining London Irish and look forward to playing in the Zurich Premiership" said Roly.
"I hope to perform well for the club and be an asset to an already great squad. Hopefully it will provide a platform from which I can regain a place in the Scotland squad."
A relieved Connor O'Shea also expressed his satisfaction at the new signing.
"Given our current injury situation in the back row with Kieron Dawson and Peter Poulos out long term, the availability of such a talented player was fortunate and we are pleased to have signed him" said O'Shea.
"We look forward to Roland joining us and making his debut next month."