Irony in Macbeth
Duncan is saying that one cannot tell someone's true nature by examining their face. This is an example of dramatic irony because he is talking about the traitorous Thane of Cawdor just as Macbeth, the new Thane of Cawdor, comes into the room to greet him. This is ironic because Macbeth ultimately betrays Duncan.
These words are filled with dreadful irony and foreshadowing. Considering how Macbeth has already imagined a floating dagger prior to killing Duncan and his current agitated state, the likelihood that one or both of them will mentally suffer as a consequence of this action is a distinct possibility.
Notice how in this speech Lennox asks several questions about the main events in the play. These are means to be rhetorical and ironic, as having witnessed Macbeth's strange behavior earlier and the poor condition the country is in, Lennox now believes Macbeth is guilty of murder and that he needs to be overthrown.
This is an example of verbal irony in which Lady Macduff says one thing but intends to be understood as meaning something that contrasts with what she says. In this case, she tells her son that Macduff is dead even though the boy knows this is not the case.
Notice the dramatic irony in this line. While the audience knows what has already happened, neither Malcolm nor Macduff yet know that Macbeth has already had Macduff's family and household killed.