Situtational Irony, she had the opposite reaction that most housewives would have had upon the news of their huspand's death.
Mrs. is informed that her huspand has died.
When she discovered that her huspand was still alive, she went into shock and later died. She was so happy upon the news of his death that when she saw him standing in her house alive and well it sent her into shock.
Dramatic irony, we know everything going on at this point while Brently Mallard is completely in the dark.