"Lamb to the Slaughter" is a very clever tale of a woman's quick response to a difficult situation.
Irony exists in the tale in Mrs. Maloney's reaction to the news of her husband leaving her. For the entire beginning of the story, Dahl paints her as a doting, overly-concerned, rather weak-minded character who lives only for her husband and his bidding. We would not expect her to brutally murder him just minutes later, but, that is what she does. It seems to surprise even her self. When something occurs that is the opposite of expectations, that is irony. After the murder, she calmly and collectedly concocts a cover-up story, even finding a way to safely dispose of the murder weapon. This behavior, compared to her previous rather anxious and weak self, is ironic.
The black humor exists especially at the end. It really isn't funny that she has killed her husband, but we can't helpt but laugh as the cops cluelessly eat the lamb. It's a rather dreary humor, but, entertaining nonetheless.
Foreshadowing is harder to find; there are few indications of Mary's behavior. When she puts the lamb in the oven, that could foreshadow the cops eating it later; all of her preparations before the mirror, practicing what she would say at the grocer's, were foreshadowing of her performance there later. It was evident that she was planing an alibi. If we read closely, we can piece together the clues of her plan beforehand.
I hope that helpd; good luck!