Dramatic irony is when the spectator or reader knows something one or more of the characters do not. In this case, the reader knows that Mary has killed her husband and is trying to get away with it. The police, doctors, and photographer have no clue. Dahl keeps the reader in suspense in this way. The reader waits anxiously to see if Mary will get away with the murder. It is interesting that some (maybe most) readers will root for Mary without realizing they are rooting for a murderer. With the detectives unaware of the fact that they are eating the evidence they are looking for, the story develops into a dark comedy. The irony is so rich (pun on being rich and tasty), it is unbelievable to the point of absurdity.
Verbal irony is when a statement's actual meaning is different from what is expressed. Verbal irony tends to be intentional. For example, when someone says that a window is as clear as mud, he/she means that it is dirty, not clear. But verbal irony can also be unintentional. If the auditor/reader/spectator discerns a meaning other than what is expressed, this also qualifies as verbal irony. In one of the last lines, one of the investigators, speaking of the evidence, says to Jack, "It's probably right under our noses." He means this figuratively; he means to say that the weapon is probably somewhere easy to find. He is correct, but he has no idea that the meat they are eating was the murder weapon. He has no idea that the evidence is literally under their noses as they eat it. This example of verbal irony is used for dark comedic effect.