How is the last line of The Story of an Hour an example of dramatic irony?

"When the doctors came they said she had died of heart disease--of the joy that kills."

This last line is ironic in that the cause of death of Mrs. Mallard had been presumed to be from the shock of finding her husband is still alive when in reality she had died from the joy of knowing that she is finally free of him.

Explanation:

Dramatic irony is the literary device used by writers to make the story captivating. it is used in speeches or in situations of a drama, where the situation can be understood by the audience but not by the characters of the drama.

In "The Story Of An Hour" the ending of the story shows Mrs. Mallard dying because of "the joy that kills". While the doctors and other characters are of the opinion that she had a heart attack in finding out that her husband, whom she's been mourning, was indeed alive. She had in fact died because of her joy in knowing that she is finally free of her husband and his "powerful will bending hers".