In Act 2, Scene III, the subject of the porter's jesting is ironic because?

Over the course of Macbeth, dreams, symbols, fantasy, and visions impinge upon the "real world." In the Porter scene, the Porter imagining that he guards the gate to Hell ironically creates a gate of “real” hell caused by regicide. When the Porter opens the gate for the thanes, he mentions that he and his friends were out "carousing till the second cock" (II iii 23). This statement calls to mind the cock that crows in the New Testament after Peter betrays Jesus by denying knowledge of him (Matthews 26; Luke 22). In Macbeth, the betrayal occurs in a more active form as Macbeth murders Duncan after the crows of the cock.