Although there are many examples of irony in A Separate Peace, two examples are:
1) In many cases, Finny has power over Gene's actions. For example, he convinces him to go to the beach, resulting in Gene failing his first test. Gene changes his mind about enlisting because of Finny's return to school. However, in the end, Gene's jouncing of the limb causes Gene to have a significant amount of power over what happens to Finny. It is ironic that although Gene is jealous of Finny's abilities, it is Gene who actually has a great deal of power over what happens to Finny.
2) In chapter 2, Finny "practically saves" Gene's life when he grabs Gene's arm when he loses his balance on the limb. This is ironic because Gene later jounces the limb, resulting in Finny's death. The name "the Super Suicide Society of the Summer Session" is also ironic because Finny ends up dying as a result of the club's actions and Gene feels as if he has died too.