Kikuchiyo claims to be a samurai, and when the others laugh at him he shows them his birth certificate as proof. This is an example of dramatic irony, as Kikuchiyo does not realize (but Kambei does) that the birth certificate states that Kikuchiyo is 13 years old.
Lord Kikuchiyo (verbal irony)
When Heihachi repeatedly calls Kikuchiyo “Lord Kikuchiyo” throughout the film, it is an example of verbal irony, specifically sarcasm. Heihachi knows that Kikuchiyo is not really a samurai or lord (and later learns that he is actually from the peasant class originally), and he calls him "Lord" to mock the fact that he is actually of much lower social standing than he claims to be.
Buddhas or something (verbal irony)
When Kikuchiyo asks the other samurai if they thought the peasants were “Buddhas or something,” it is another example of sarcasm, a type of verbal irony. He means to say that the peasants are actually the opposite of (or at least quite far from being) Buddhas in their morality. He uses sarcasm both to criticize the peasants and to criticize the samurai’reactionon to the peasants’ misdeeds.
Yohei defends his post (situational irony)
When Yohei defends his post courageously after Kikuchiyo has abandoned it, we can consider this an example of situational irony. For much of the earlier parts of the film, Yohei is particularly frightened and prone to cowering or giving up. Kikuchiyo specifically has to repeatedly force Yohei back into line when he tries to flee or is cowering. We do not expect him to be the one that ends up courageously defending his post from an attack, and his behavior contrasts with these expectations. This example of situational irony is used to criticize Kikuchiyo for his selfish abandonment of the post.
Seven Samurai Questions and Answers
The Question and Answer section for Seven Samurai is a great
resource to ask questions, find answers, and discuss the novel.