I think one good example of irony in the book is the name "Super Suicide Society of the Summer Session." It is an ironic name because because Finny ends up dying as a result of the club's actions. Afterward, Gene feels as if he has died too. The club's name winds up not being such an innocent and harmless name.
A second example of irony within the book is the character of Leper. He is portrayed as a weak character, and he is the character most likely to fight in the war only if drafted; however, Leper becomes the first boy to join the war effort because he volunteers to join.
A third example of irony is that Gene is the person who bounces the branch, which ultimately leads to Finny's death. It's ironic because Gene is supposed to be Finny's best friend. Finny trusts Gene more than any other boy at the school, and it's unexpected to readers that Finny's most best, most trusted friend winds up being the cause of his death.