"There are no reasons" (dramatic irony)
Renton claims that there are “no reasons” he chose against life, and chalks his decision up to a heroin-fueled nihilism. However, the audience understands, from the tone of his opening monologue, that Renton is in fact experiencing a relatively normal youthful phase of disillusionment with society. Though he later will change his perspective, it seems that at this juncture he thinks his situation is different from what the audience knows it to be.
“No way would I poison my body with that shite” (dramatic irony)
Begbie says this while holding a glass of whiskey and smoking a cigarette. He thinks that he keeps his body healthy because he does not use heroin, but the audience understands that even while he criticizes Renton for poisoning his body, Begbie is also poisoning his own body with different substances. Additionally, he thinks he has the moral high ground because he does not use heroin, but in reality, the audience knows that his violent actions are more reprehensible than Renton’s actions.
Spud gets too drunk for sex (situational irony)
Spud’s complaining in the club about not having sex with his girlfriend (Gail), and Gail’s discussion with Lizzie about wanting to have sex, gives the audience an expectation that Spud and Gail will finally have sex after the club night. Additionally, we understand that the friends decided to go to the club with the specific hope that the night would end with sex. However, despite Spud’s apparent longing to finally have sex with Gail, he gets too drunk and entirely misses his chance. Though the audience knows that Spud is fairly incompetent, it is still relatively early in the film, and we do not realize that he will screw up the situation so badly, especially considering how impassioned he is when discussing the subject with Tommy.
Diane turns out to be underage (situational irony)
When the audience is introduced to Diane, she is far and away the most confident and clever character we have met. She can drink heavily without issue (we see her rapidly down two drinks that a man offers her), she knows how to respond wryly to Renton’s sexual advances, and she treats him with a mean coolness that establishes her dominance in the situation. In her apartment, she maintains control of the situation, confidently retrieving a condom and undressing, prompting Renton to undress as well. The audience certainly does not expect her to turn out to be a 15-year-old high school student—if anything her maturity and confidence might lead the audience to expect her to a normal (as compared with Renton) adult. When we find out that she is a minor, it contrasts with our expectations of what was going to happen, and generates an understanding of Renton’s confusion and shock.
