No free drinks
In the opening verses of John 2, Jesus turns water into wine for a wedding feast in Cana—the first of his many miracles, and the most Irish of the lot. Mulligan's Joking Jesus unhospitably demands faith as the price of admission to such alcoholic miracles: those who think "that I amn't divine" will "get no free drinks."
In addition to threatening nonbelievers with the pains of sobriety, Joking Jesus rather fantastically condemns them to drink urine, by a pun that Mulligan has already put into the mouth of Mother Grogan. They will "have to drink water," but since to "make water" is to urinate and since Jesus seems to be making all the drinks in the place, the water they drink will be the stuff "That I make when the wine becomes water again." They may "wish it were plain"—i.e., Guinness' stout, as in the Irish phrase "a pint of plain"—but this water is done with being miraculously converted.