I thought she was going to be sad because her husband didn't love her anymore. He was starting to act differently.
""Sit down," he said. "Just for a minute, sit down." It wasn't until then that she began to get frightened."Go on," he said. "Sit down." She lowered herself into the chair, watching him all the time with large, puzzled eyes."
The irony happens when they eat the lamb. They take a break from searching for the murder weapon to eat it. This is dramatic irony
'"It's probably right under our noses. What do you think, Jack?"'
The irony leaves a feeling of eeriness because Mary is going to get away with killing her husband and she is giddy about it.