The incident took place at the Black Forge Inn in Drimnagh Road, Dublin on Wednesday night.
The former UFC champion, 33, had been at the pub earlier on, according to social media posts.
A source told Dublin Live that two men on a scooter were spotted headed towards the pub before the alleged attack.
Gardai are appealing for witnesses to come forward, saying: “Gardai are particularly keen for anyone with camera footage from the area overnight to make this available to them.”
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