Irish, who will join the English Premiership next season after winning promotion, announced the signing of the Test centurion on social media on Monday night.
Kepu has been a mainstay of Australian Test rugby and the Waratahs for more than a decade, playing 135 Super Rugby matches and 103 Test matches.
Waratahs coach Daryl Gibson said Kepu would leave a hole at the Waratahs, but also had provided a strong foundation for the next generation to develop.
