Irony is a literary technique, originally used in Greek tragedy, by which the full significance of a character's words or actions is clear to the audience or reader although unknown to the character.
The irony in the details is that they build an optimistic mood, while the reality is that Mrs. Mallard just received the news that her husband died, news that aren't optimistic at all. The reader would expect the details to be dark and sad to reflect the feelings of a sad widow. The death of her husband starts a life that she can live just for herself. She feels that she has just been reborn in a way. There is the irony.