It is ironic because Oedipus was his actual murderer.
In "Oedipus Rex" by Sophocles, Oedipus can only end the pestilence in his kingdom by identifying and punishing Laius' murderer. Laius was his father (though he was unaware of the fact), his wife's first husband, and he also preceded Oedipus to the throne.
Oedipus' people, the Thebans, are not being as helpful as he wants. He sets a punishment for anyone--no matter who the person is--who knows the murderer and does not come forward, as well as anyone who helps the guilty.
However, he was the murderer unknowingly. Oedipus and Laius had been involved on an accident at a crossroads without knowing each other's identity. Enraged because Laius almost ran him over, he had attacked and killed him. So since he is in fact the murderer, he was actually cursing himself.