Keatley, 33, won seven Ireland caps between 2009 and 2017 and was most recently with Benetton.
Fly-half Adam Hastings, who will join Gloucester from Glasgow in the summer, is currently injured.
"It's a strange time to be joining the team with everything that's going on with Covid," said Keatley, who registered 1,247 points for Munster.
"However, I'm really happy to be joining a club and a city with a proud rugby history."
Head coach Danny Wilson said: "Ian's experience will be great from a performance point of view.