Huck is a born trickster himself and the irony is that he cannot understand that a trick is being played on him. The man on the horse is really just pretending to be drunk, just as Huck pretends to be many things on his trip down the river. This episode is included while the king and the duke prepare to trick the town into watching their "play". So Twain doubles the irony. Huck can't tell that he is being tricked while he knows the king and the duke are preparing their trick.