The title itself is using verbal irony. Normally when someone says "lamb to the slaughter" it refers to a lamb that is unknowingly going to be killed. That is not the case with this story. The phrase "lamb to the slaughter" this time means a lamb is coming to do the actual killing. I keep hearing in my head something like "superman to the rescue," but with "lamb, to the slaughter!"
Another use of irony is dramatic irony. That's the kind where the audience knows something the characters do not know. In this story that occurs at the end when an investigating officer states that the murder weapon is likely just under their noses. He doesn't realize that the murder weapon is very much indeed under his nose, since--because of Mary's brilliant presence of mind--he is eating the lamb that was used to bludgeon the victim, Patrick, to death.