Aye I thought it was a punch at first but watching again the bloke's reaction and the glass still being intact in his hand has me thinking he's took a sip and then hoyed the drink in the fella's face.
Imo, McGregor is gesturing with his left hand telling him to get outside, then strokes his beard and as the old fella looks away he punches him in the side of the head.
Plus the old fella's head snaps back, which it wouldn't do if he had a drink thrown on it when he wasn't looking.
And if you zoom in on the left picture here you can see no glass