Final answer:
The irony in Mrs. Mallard's death by 'the joy that kills' is the mistaken diagnosis, as it was actually the loss of freedom that caused her heart to fail.
Step-by-step explanation:
The irony in the final statement that Mrs. Mallard died of 'the joy that kills' in 'The Story of an Hour' lies in the unexpected turn of events. Throughout the story, Mrs. Mallard experiences a sense of freedom and relief after hearing the news of her husband's death, only to have him return alive. The joy referred to by the doctors is mistaken; it was the shock and disappointment of losing her newfound freedom that led to Mrs. Mallard's fatal heart attack, making the diagnosis of 'the joy that kills' an example of dramatic irony.