The 31-year-old former UFC featherweight and lightweight champ first quit the sport in April 2016.
In March 2019, he again announced he was hanging up his gloves, but on both occasions the retirements were short-lived.
"Hey guys I've decided to retire from fighting. Thank you all for the amazing memories! What a ride it's been!" the Irishman said, adding a picture of him and his mother and promising to buy her a dream home.