Play video content Exclusive TMZSports.com. Conor McGregor uncorked a shot to the head of an elderly man in an Irish bar in a dispute over whiskey -- Conor's whiskey -- and TMZ Sports got this video showing the violence.
Conor walked in and lined up cups for bar patrons ... looking to buy a round of his Proper Twelve for everyone.
Conor placed a cup in front of him not once, but twice ... and he still refused.
Unclear if words were exchanged between them, but after Conor downed a shot with the other drinkers, he unexpectedly threw his notorious left fist right in the face of the old man who rejected the booze.
Perhaps most odd of all ... the poor, old pub patron who got socked didn't even get knocked off his stool.
The video is just the latest evidence of McGregor losing his cool and acting like a thug outside the Octagon.
Just a thought, but Dana might wanna sign that old man from the Dublin pub. »