The irony of competence and birth rank
Because Korea culture at this time was to spoil the firstborn son and spur the other children, there is an implicit irony in this novel that appears almost immediately: the rejected son is the more competent, because his life was inherently more difficult, while the firstborn son was spoiled.
The irony of nobility
In this novel, true nobility is ironic, because the best character in the book isn't actually that noble as a person. He is a thief who steals from the rich and gives it to the poor, but his nobility is clear. Where does that nobility come from? It comes from his confidence in himself, regardless of the wrongful opinions that his father believed about him. Ironically, Gildong ends up being the best king in the story, even though he's a criminal.
The irony of good thieves
The rag-tag group of thieves who operate under Gildong's leadership are called "Hwalbindang," which simply means "Those who help the poor," basically. They are ironic because thieves typically represent selfishness, because they do something dishonorable to get something, but actually, this group gives all their money away to the poor, and they only steal from the exorbitantly wealthy. So they represent the opposite of greed.
The irony of mastery
Although Gildong was a frustrated, mistreated youth, and although much of his psychology seems to be specifically shaped by his anger at his father, the whole endeavor had an unexpected benefit. He became a master. His physical mastery saves his life again and again, both in combat and otherwise. Also, his mental mastery makes him a literal wizard who can do magic. All because he was not allowed to have what he wanted: his father's approval. Ironic.
The irony of status
To Gildong, his frustration about preferring the firstborn goes way beyond birth rank. He typically sees the social caste system as a mistake, perpetuated by human greed. He is happy to be considered low status, and he is a hero of the underdog, stealing from the greedy overlords and giving to the people in the lower class. He doesn't believe people are more or less valuable just because of how much money they have.
