Barty
Barty had no idea it was Kirley's birthday until it was too late, but makes a mental note to find out what he likes and pick him up something later on.
He wouldn't drink a whole lot, just enough to keep him occupied and keep him from doing something stupid like drunk dancing (though if everyone else was doing it!...). He'll chat with whomever wants to chat with him, particularly Lorcan and Bella (I think despite knowing her age he was still a little shocked to see her so young), and would have been polite and friendly to anyone else who talked to him.