One of the biggest ironies in the play is that Christy only becomes the man the villagers originally thought him to be right at the end, when he's threatened for appearing to kill his old man. Then, for the first time, he shows real courage in the face of adversity, expressing a willingness to die for his alleged crime and also to shed the blood of as many others as he can before he does.
What's doubly ironic is that when Christy first arrived in the village, spinning the tall tale about murdering his father, he was hailed as a conquering hero. But once his subterfuge has been exposed, his attempts to kill Old Mahon for real are greeted with revulsion. This would indicate that the villagers will only accept such a killing if it can be suitably romanticized and therefore made palatable.