Lealiifano on Monday night announced he would link up with theĀ Belfast-based club for the next five months before returning to pre-season training with the Brumbies in mid-January.
The deal is designed to ensure the playmaker gets more game time after he missed nearly all of the recently-completed Super Rugby season while recovering from his illness.
"I feel that it's important that I get some minutes playing competitive rugby ahead of the 2018 Super Rugby season," Lealiifano said in a statement.