In Roald Dahl's "Lamb to the Slaughter," the greatest irony in the story is how Mary Maloney literally gets away with murder with the help of the police—and they never realize it.
Generally, irony is the difference between what one expects to happen and what really happens.
Irony makes use of hyperbole, sarcasm, satire, and understatement.
Mary Maloney is pregnant and a devoted wife. One evening her husband, a policeman, comes home from work and tells his wife that he is leaving her. Much more concerned with himself (and how this will affect him at work), he is unaware that his wife is so stunned.
Almost in a daze, Mary goes to the basement to get something from the freezer for dinner. Even as he hears the sounds of preparation for a meal, Mary's husband tells his wife he doesn't want anything to eat—but he gets much more than food: for as he gazes out the window, Mary approaches him from behind and bashes him in the head with a frozen leg of lamb, killing him. There is some irony here, too: he doesn't want food; she doesn't give him food—but uses the "food" to kill him.
As he lies on the floor, Mary worries about what will happen to her if she is caught. Leaving his body where it has fallen, she puts the lamb in the oven to cook. Then she runs to grocer as if she needs more things to make an unexpected dinner for her husband (since they usually go out for dinner on Thursday nights). This provides her with an alibi. Returning home, ostensibly to find her husband murdered, she calls the police. They arrive and start looking for clues. Unbeknownst to them, the murder weapon is not only being cooked in the oven, but it will eventually be their dinner. There are several forms of irony at the story's end. First, one officer imagines that the weapon might be right there in front of them. (This is dramatic irony: Mary understands the irony, but the policemen do not.)
One of them belched.
“Personally, I think it’s right here on the premises.”
“Probably right under our very noses. What you think, Jack?”
The other example of irony (called "situational irony") is that the men have disposed of the murder weapon themselves. This is not what one would expect the police to do: and they do it with no knowledge that they have done so.
The woman stayed where she was, listening to them speaking among themselves, their voices thick and sloppy because their mouths were full of meat.
There is irony, too, in that the men not only eat, but that they finish the meat: as a favor to Mary, for she tells them she couldn't eat a thing. Dahl's story abounds with instances of irony, which make it darkly entertaining.