Irony of Martin
Martin enlists Simon's help to get Abby to like him, and frames that action as Simon being a good friend to him. However, Martin acts abysmally during these interactions, first blackmailing Simon with his sexuality, and then outing him to the whole school. While Martin might claim to be starting a friendship with Simon, he acts so terribly to him that "friendship" couldn't be a worse term for defining their relationship.
Irony of the Casanova Book
Blue's father gives him a Casanova book for his birthday, and makes a joke about how it'll help Blue get girls to like him. Ironically, Blue is gay and is planning to come out to his father, a fact that he and Simon laugh about over email.
Irony of Abby
Abby is new to Creekwood at the start of the novel, and doesn't really know anyone. Ironically, she quickly becomes the center of attention -- she joins Simon's friend group and has a number of male students who have crushes on her -- and ascends beyond her unknown status at the start of the year.
Irony of Martin's Revenge
When Simon is unable to get Martin to go out with Abby, Martin gets revenge by outing Simon on the school Tumblr. However, rather than Simon becoming a social outcast because of this, Simon gets mostly positive reactions from everyone in the school, and it's Martin who is ostracized, for violating Simon's privacy.
Irony of Blue
Blue feels mysterious to Simon, since they have never met in person and Simon has no idea what he looks like or acts like in person -- except he does, because they're in some of the same classes and see each other every day at school. While Blue might feel like someone from a different world to Simon, he is ironically under Simon's nose for the entirety of the book.
