Devlin and Alicia's Mistrust of One Another (Situational Irony)
All it would take for Alicia to turn down the job seducing Sebastian would be for Devlin to profess his love and forbid her from doing so. However, Devlin does not feel confident enough in Alicia's love for him to speak up. This leads to her taking the job and seducing Sebastian, and to Devlin planning to leave Rio. The irony is that their mistrust of each other is what, in fact, leads to a situation in which the grounds for their mistrust (Alicia's looseness, Devlin's lack of interest) are realized.
Alicia tells Devlin that she is hungover when she is in fact poisoned (Situational Irony)
When Alicia goes to visit Devlin at the park bench, she is becoming more incapacitated by the regimen of poison that Sebastian and his mother have her on. Rather than tell Devlin that she is not feeling well, she accepts her poor physical condition and lies to Devlin that she was drunk the previous night and so now has a hangover. For the entire movie, the reason that Devlin has had trouble trusting Alicia is because he believes that she is incapable of change and that she is a drunk. Here, in the moment that she is not actually drunk at all, and is in desperate need of Devlin's help and care, Alicia makes up a story to discourage Devlin from taking her to a doctor. The irony is that if he took her to a doctor then and there, she would be saved from the slow murder that Sebastian is enacting on her. However, she is too prideful to accept help from Devlin and lies.
Madame Sebastian's Mistrust of Alicia (Situational Irony)
Upon meeting Alicia, Madame Sebastian is exceedingly suspicious of the girl and doesn't believe that she is marrying Sebastian for the right reasons. While she is, of course, correct, Madame Sebastian has no idea that Alicia is in fact an American spy. While her suspicions are accurate insofar as Alicia is not marrying for the right reasons, the specific reasons she mistrusts Alicia—her general mistrust for any woman her son loves, her belief that Alicia is marrying Sebastian for his money—are off the mark. When Madame Sebastian discovers that Alicia is a spy, she is just as surprised as Sebastian, in spite of having been suspicious of Alicia from the start.
The poison (Dramatic Irony)
One of the central instances of dramatic irony in the film is when Sebastian and his mother begin to poison Alicia. Having been privy to their conversation plotting to poison her, the viewer knows that they know Alicia's real identity and will murder her in the coming weeks. Furthermore, Hitchcock broadcasts to the audience that they are poisoning Alicia's coffee by photographing the coffee in close-up, which serves to communicate to the audience that this is their means of killing her. For a time, Alicia is completely unaware that they are on to her, or that they are poisoning her coffee, but the audience knows. This creates a tense atmosphere of dramatic irony, in which the audience knows a salient detail that the character does not.
Sebastian is more gregarious than Devlin (Situational Irony)
Another irony of the film is the fact that Sebastian is so devoted to and loving towards Alicia. In spite of being a Nazi sympathizer, a demonstrably evil and traitorous man, he dotes on Alicia with a sweet affection and respectful attention. The viewer senses his genuine love for her, whereas Devlin, the man on Alicia's side who is also very devoted to her, is unable to demonstrate his love. Devlin is a markedly cold man, and for the majority of the film he is unable to express his feelings to Alicia, which makes her feel badly. By the end, he has transformed into a heroic romantic knight-in-shining-armor, and Sebastian has begun mercilessly poisoning his wife to get her out of the way. For much of the movie, however, Sebastian is a soft-spoken and charming lover despite being a Fascist, and Devlin is a brusque, disrespectful, and repressive bully, in spite of being on Alicia's side.
