the 31-year-old MMA fighter landing a blow on the drinker, in his 50s, before being ushered away by pals.
The unflustered victim barely moves after the attack and remains sat on his bar stool.
It is understood McGregor lashed out after the man turned down a free drink from the star, who had offered his own brand whisky to everyone inside the venue.
Footage shows McGregor entering the pub and walking up to the bar before the incident took place.
McGregor has spoken to detectives about the incident but was not formally arrested over the matter.
A file has been passed by police to Ireland's Director of Public Prosecutions to make a decision over whether to charge the star.
A Garda spokesman said: “(We) are investigating an incident at a pub in Drimnagh, Dublin, on April 6.
"No arrests have been made. Investigations are ongoing.”
Last year, McGregor was convicted of disorderly conduct after attempting to confront Ultimate Fight Championship rival Khabib Nurmagomedov on a tour bus in New York.
McGregor ran alongside the bus, grabbed a metal trolley and threw it, shattering the bus's window.
He was sentenced to five days of community service and to attend anger management classes after appearing in court in New York.
In March, CCTV footage emerged of McGregor stamping on the mobile phone of a fan in Miami.
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