Irony abounds in Kate Chopin's "The Story of an Hour." For example, after Louise retreats to her room, her sister Josephine knocks on her door and begs her to come out so that she does not suffer alone locked away in her room. At this point, however, Louise is incredibly happy because she has started to realized all the freedom that she has now that her husband is dead. This is an example of dramatic irony--the reader and Louise are privy to information that Josephine is not.
Similarly, the last line of the story employs dramatic irony by suggesting that Louise has died from "a happiness that kills." The reader has already come to an understanding that Louise's life is better without her husband, so when he shows up perfectly alive, Louise's sense of freedom is taken away. She dies because she feels again imprisoned, not because she is happy.