Irony comes into play at the end of Chopin's short story. Throughout the story we've heard how Mrs. Mallard's husband died in a train accident. She cries then goes to her room to collect herself, or so her friends think. While she is in her room, she thinks about her life and what it will be like now that she no longer has to "bend" to someone else. She thinks about her life and how she is now "Free! Free! Free." While hugging herself with the excitement that life now offers her, her sister, Josephine, pleads to be let in. Josephine knows her sister has "heart trouble" and is worried that she will cry herself sick. The dramatic irony occurs when we, the audience, know that this is not the case. In fact, it's quite the opposite.
When Mrs. Mallard finally emerges and glides down the stairs, the door opens. Who can it be? Mr. Mallard, her husband! He is very much alive and was actually nowhere near the accident. Josephine gasps as her sister collapses to the ground. The doctors decide that it was her heart; the weakened Mrs. Mallard's heart was too happy to see her husband, and so it gave out. We know the truth. She was not happy that her husband was alive, and so she died instantly. Quite the opposite of what one would expect to see when they find out their husband is alive.