Janis' Plot (situational irony)
Janis uses Cady to infiltrate the Plastics and get revenge on Regina. Although Janis justifies her actions by maintaining that Regina is evil, Janis's methods seem evil as well, leading to an irony that is difficult for Cady and the audience to evaluate early on in the film.
Cady's Innocence (situational irony)
Cady's innocence ironically makes her an appealing addition to The Plastics' group, which is defined by its cynicism, given that Regina likely thinks she will be able to manipulate Cady at will. Cady's naivete prevents her from understanding Regina's motivations. But later, it will turn out that Cady's "innocence," which made her seem a candidate for manipulation by the cynical Plastics, has actually allowed her to quickly learn their techniques and manipulate them in turn. Ironically, her naivete is what leads her to become (for a moment) the most cynical and knowing of them all.
Cady's Halloween Costume (situational and dramatic irony)
Cady wears an ornate "zombie-bride" costume because she ironically does not realize that modern teenage norms stipulate that girls should wear simple, sexy dresses to Halloween parties, rather than actually scary outfits.
Regina Writing in the Burn Book (situational irony)
The audience expects that Regina will place Cady's photograph next to the obscene note she has just scrawled in the Burn Book, after learning that Cady has been feeding her weight gain bars. Instead, Regina places her own photo there, puzzling the viewer until the next scene reveals the scope of Regina's scheme.
