Dramatic irony means that the audience knows something the characters do not. For example, in the beginning of Hamlet, we know immediately that Claudius was the one who killed Hamlet's father, however, he doesn't know that from the very start.
Sometimes, sunset can be viewed as "romantic". If the author wants to build a relationship the sunset is used as a pathetic fallacy (when the weather matches the mood). The sunset refers to love which will overall build their relationship.
