There are many examples of irony in "Lamb to the Slaughter," but the penultimate line of the story is exceptional in providing examples of situational, dramatic and (arguably) verbal irony all at once. The line is:
Probably right under our very noses. What you think, Jack?
The situational irony here could scarcely be stronger: the policemen are eating the evidence for which they have been searching so assiduously. The dramatic irony is similarly strong: the reader knows how Mary's husband was killed and shares her amusement at the unintentional aptness of the policeman's comment.
Finally, there is an unusual variety of verbal irony in the phrase "right under our very noses." This is not sarcasm, since the policeman does not intend to be ironic, but he uses what is intended to be a non-literal phrase with ironic exactness. When we say something is right under our noses, we mean it is in plain sight, not that it is literally immediately beneath the nose. The mouth, however, is literally beneath the nose and the officers have been chewing the murder weapon as they discuss its absence.
There are, of course, more traditional examples of verbal irony in the story, including the title, which suggests that the lamb is a sacrificial victim rather than a bludgeon.