Question

What is ironic about the ending of the play?

Oedipus, it was prophesied when he was a baby, would kill his father and marry his mother. So he was cobbled by his feet and exposed as an infant. The commoner charged with exposing the prince, however, was childless and took pity upon the babe, and reared the child himself. Oedipus as a young man learned by the same prophesy that he would become a patricide and enter into an incestuous marriage with his mother. So he fled what he thought were his true parents, home, and birth.

Had Oedipus not tried to flee his fate, he would never have met his father the King in the road and killed him in an argument. Had the future king of Thebes not tried to flee his...

And he would never have blinded himself, injuring his body as his father tried to kill his own child so many years ago by injuring the baby's foot -- thus resulting in the name of Oedipus 'swollen foot.'
Oedipus' hubris in thinking he could avoid his fate and defy the god's will echoes his father's willingness and hubris to attempt to do the same. Even Oedipus' freeing of the city of his birth from plague is ironic, as his attempt to do so merely brings another, worse plague to the city later on. Thus, the entire play's plot is an exercise in irony as well as a demonstration of the main character's hubris.