He said that the baby and his mother are doing great.
Conor McGregor has announced the birth of he and partner Dee Devlin's third child.
Rían McGregor is the baby's name, which McGregor confirmed on his Instagram page on Monday afternoon.
He said: "The McGregor Clan is now a family of five. Healthy baby boy delivered! Baby and Mammy Wonder Woman are doing great! God I thank you for everything you give to me and my family in this world."
Both Dee Devlin and Conor McGregor have been in Dubai with their two children, Conor Junior and Croia, after his last UFC outing in January - a defeat to Dustin Poirier.
McGregor will now take on Poirier in a rematch later this summer.
The fight will take place on July 10 at UFC 264 at the T-Mobile Arena in Las Vegas at 100-per-cent capacity.
Dana White confirmed this news earlier in the year, saying: "Ladies and gentleman, that's 20,000 fans."
Poirier defeated McGregor convincingly in January of this year, in what was already a rematch after McGregor defeated the American in 2014.
Just last week he celebrated being Forbes' highest paid athlete in the world, saying: "It was a dream to lead this list with so many successful athletes."