We know who Karen Eiffel is before Harold does (Dramatic Irony)
From early on, we can hear Karen Eiffel's voice narrating Harold's life, and oddly enough, so can he. Soon enough, however, we are introduced to the character of Karen Eiffel, a successful tragedian who is suffering from writer's block while working on her new novel, which just so happens to be about Harold Crick. We see that she is concerned first and foremost with researching her novel. As Harold struggles to piece together who the disembodied voice who prophesied his death is, the viewer knows it is Karen Eiffel before Harold does. Thus, this is an instance of dramatic irony, in which we know something before Harold does.
Karen Eiffel doesn't know Harold is a real person (Dramatic Irony)
Just as Harold has no idea who Karen Eiffel is, Karen Eiffel has no idea that Harold Crick, a man whom she thinks is a figment of her imagination, is a real person. She writes about her character, trying to make him as realistic as possible, without having any idea that he actually exists, and that she has power over his actions and his life. This is another instance of dramatic irony, in that the viewer knows that Harold is real well before Karen has learned about it.
We know that Karen is going to kill Harold before he does (Dramatic Irony)
Not only do we know that Karen Eiffel exists and who she is before Harold does, but we learn of her intentions to kill him off in her novel (and by extension, reality) before he has any idea. When we first meet her, she is plotting and researching different dramatic ways to kill her character, and we quickly learn that the person she wants to kill is Harold Crick, our protagonist. Thus, there is a period of time when we know that Karen Eiffel plans to kill Harold—and thus, given the film's logic, that Harold is going to die—but he has no idea until he hears her narrate: "Little did he know that this simple, seemingly innocuous act would result in his imminent death." The parts when we know more about Harold's fate than he does are an extended instance of dramatic irony.
The watch that was supposed to kill him ends up saving his life (Situational Irony)
While Karen Eiffel set out to write a novel about a man whose fastidious attention to time and details ends up indirectly killing him, at the last minute, she changes the ending to have a happy ending. The fact that Harold's watch is set to the wrong time was originally supposed to be what killed him. However, after her change of heart, Karen writes the story so that a piece of the watch gets lodged in Harold's arm, preventing him from losing too much blood and saving his life. This is an instance of situational irony, as Karen Eiffel is known for killing off her characters in especially affecting ways; it is a complete departure from what is expected that she chooses to spare Harold.
