One of the reasons this story is so successful is that its title works on many different levels, and part of the evident irony of the story is the way that this title functions. Let us consider the main meaning of this phrase. "Lamb to the slaughter" is used to describe someone who is normally innocent, weak and defenceless going into a situation where they are going to get attacked, beaten up, or killed (metaphorically if not physically). You could argue that it is Mary Maloney who is going to get "slaughtered" by the news that her husband is leaving her. She is clearly a well-meaning innocent. However, equally, and more deviously, it is Patrick Maloney who comes like a lamb to the slaughter. He would never have expected his wife to kill him (and nor would the reader) and thus goes like an innocent to his death. Of course, the irony is compounded by the fact that lamb is literally used to slaughter Patrick. Likewise it could be said that the policemen who come go like lamb to the slaughter - they are intent on finding the murder weapon to find the killer, but are innocently deceived by Mary Maloney into eating the very murder weapon that they are talking about needing to find. They are poor, unsuspecting little lambs who are metaphorically "slaughtered" or tricked by Mary into getting her off the crime free.
Thus the fact that the "lamb" could be identified as any one of the characters combined with the actual use of the leg of lamb as the murder weapon serves to make this a highly successful ironic title that operates on many different levels.