To do 'The Dirty Guinness' one must down one's pint of Guinness then continue to thrust said empty pint glass into ones face, shattering it in the process.
Paddy O'Halloran: I can't make it into work today boss.
Boss: Why might that be Paddy?
Paddy O'Halloran: I only went and did The Dirty Guinness a few times last night, it was Finlay's stag do!
Boss: Good lad Paddy, I'll be seeing you tomorrow then.
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.