According to TMZ, Conor McGregor went to a Dublin Pub in April and wanted to buy a round of his whiskey brand Proper Twelve. Well, this aged gentleman didn't want any. McGregor puts the glass in front of him twice, but the man refuses both times and puts the glasses back behind the bar. So OBVIOUSLY McGregor punches him. Like any totally sane adult would do.
The best part, though, is that the guy barely even reacts to McGregor's punch. Takes the punch, doesn't get upset. Just slowly turns back around and is like "lol what's that guy's problem." What a legend.