The incident is said to have occurred in April in The Marble Arch pub in Dublin, with the website TMZ Sports recently releasing the video.
In has been reported the 31-year-old was attempting to buy a round of his own 'Proper Twelve' whiskey for everyone in the pub.
However, the older gentleman twice declined the offer from the former UFC featherweight and lightweight champion, which led to McGregor violently lashing out and hitting the man in the head, who managed to stay on his feet.
The fighter, who had just downed his drink, then appears to be held off from committing any further offenses and is carried away by two other men.
It's not clear what happened after the incident but the video has reportedly been handed to Ireland's Director of Public Prosecutions by the police where a decision will be made as to whether the star will be charged.