The irony of the tragic ending
The reader might want the character to escape the novel in his pure, just form, but as an indication that he is a real person, McCarthy shows his dark side at the end. Traditionally identified by his willingness to help suffering animals, she shoes the dog and learns the terrible lesson of evil and its consequences in the soul. When he sees the dog has escaped, he mourns bitterly in a fit of weeping.
Boyd's ironic fate
Boyd literally got a second chance, but he fails to change his ways and instead repeats his mistakes, offending another person and getting himself shot. Ironically, he dies by doing the one thing he should know perfectly well not to do. After all, he was literally shot—not even that trauma was enough to make him change his stubborn ways.
The irony of random violence
No one expects how dark and malevolent some people on the earth can be, so as a teenager transitioning into his adulthood, Billy must learn the difficult truth that not only is life difficult and arduous, there are also people who are dedicated to evil, making it all the more difficult. However, Billy survives and a mystic gypsy is there to help him. The universe seems to be letting Billy get a glimpse of the darkness, but just so he knows it's there. At the end of the novel, he learns why people turn evil, because he makes an evil decision and finally gets the bigger picture about human evil and its roots in suffering.
The irony of the threshold
Another thing that children take for granted, that Billy has to fix before he can be an adult, is that the world may seem small and simple unless you travel beyond your comfort zone. By taking a risk and accepting a quest into the dark unknown, Billy learns that life is infinite and complex, and perplexing and beautiful. His "crossing," to borrow the title, is the crossing into the land of the unknown potential of human existence. He's doing the most ironic thing one can possibly do—foregoing his personal safety in hopes of attaining a higher good.
The irony of the father
Instead of showing the father as a tyrant, or as a golden hero of some kind, the narrative depicts Billy's father as a rather thoughtful, intelligent man who can solve problems without violence. Billy's character is the flower, but his father's integrity is the soil. Ironically, Boyd's perspective is different. He sees the worst of his father and becomes bitter. What changed? The character and attitude of the son.
