These are great examples. The moment of irony I always enjoy is in the last act, when Malcolm and Macduff and old Siward, with their army, are closing in on the castle. They're hacking down tree branches for cover, fulfilling the prophecy of a moving Birnan Wood on the way to Dunsinane Hill and thus demonstrating to the audience that, without a doubt, the other prochecies are not true. While the forest is moving, Macbeth is shrugging off the armor someone brings him and bragging that he has nothing to fear. We know better, and that's dramatic irony.
In the witches' first greeting to Macbeth, there is dramatic irony. When they hail him as "Thane of Cawdor," Macbeth thinks this is a prediction, but in actuality King Duncan has already named him as such. Only their remark "All hail, Macbeth, that shalt be King hereafter" is a true prediction.
Dramatic irony (as other editors have noted) occurs when the audience knows more than the characters onstage, and as such, is a wonderful device to create suspense. When an audience knows that a character is walking into a trap, the suspense is very high.
For me, most of the true suspenseful moments come in the early acts of the play. Shakespeare does a great job of building the suspense through the murders of Duncan, Banquo and the surprise appearance of Banquo at the banquet.
A subtle, but very well developed, moment of dramatic irony and suspense is the beginning of Act II. Just prior, at the end of Act I, Lady Macbeth has just convinced Macbeth that the murder will take place. What will happen next? The audience is on pins and needles. Act II, scene i begins with Banquo and Fleance onstage. They are joined by Macbeth, who enters, calling himself "A friend." Though he has not yet plotted Banquo's murder (that we know of), this is a nice ironic touch by Shakespeare. Banquo then goes on to emphatically describe how happy and content Duncan is to be housed by so gracious a host and hostess -- IRONY!
And, after telling Banquo that he has something he wants to discuss with him (again, possibly a moment that arouses the audience's ironic suspense as to what will happen next), Macbeth wishes Banquo "Good repose the while." Also, IRONY. Who will sleep well in the hours to come, what with all the murdering and knocking and shouting?
This scene is but one example of how dramatic irony can lead to a great sense of suspense in the expectations of the audience.
In Shakespeare's Macbeth, dramatic irony is used in several scenes of the play. Dramatic irony refers to situations that are known to the audience (and other characters) but remain unknown to the characters in the story. The audience and Lady Macbeth know that Macbeth is responsible for the murder of King Duncan; however, none of the other characters are aware of his involvement. This allows the audience to watch the downfall of Macbeth's character while he tries to maintain a "false face" in front of the other characters. Dramatic irony also helps build suspense as the characters begin to be suspicious of Macbeth. By the end of the play, Macbeth's crimes are revealed and he suffers the consequences of his actions.
Dramatic irony occurs when the audience knows more than the characters. The earliest example of dramatic is our knowledge that Macbeth has been named the new Thane of Cawdor before he knows it. When Ross delivers the news to Macbeth, we are awaiting his reaction. We know that part of the witches' prophecy has come true.
Later we know that Macbeth and Lady Macbeth are plotting to kill Duncan. Duncan upon riding to their castle comments on its pleasant air. His statements are almost comical in light of the evil scheming being done within. This dramatic irony ties in nicely with the appearance versus reality theme that governs the play.
Another poignant example occurs in Act 4 with the death of Macduff's wife and children and servants. We know long before Macduff is aware of it. When Ross first declares to Macduff that his family is "at peace," the reader knows what that phrase really means. We know the hearbreaking news that Ross will deliver to Macduff, and we dread, but watch with fascination, his resulting anguish.