In Macbeth Act 1, Scene 3, what is an example of dramatic irony?

The reader knows that the witches are going to mess with Macbeth, but Macbeth does not.

Dramatic irony occurs when the reader or audience knows something that the characters do not.  In this case, the witches are discussing how they will mess with Macbeth.  The audience or reader knows what will happen, but Macbeth does not.

In the beginning of the scene, the Weird Sisters describe their actions as witches and how they are going...

The reader knows that the witches are going to mess with Macbeth, but Macbeth does not.

Dramatic irony occurs when the reader or audience knows something that the characters do not.  In this case, the witches are discussing how they will mess with Macbeth.  The audience or reader knows what will happen, but Macbeth does not.

In the beginning of the scene, the Weird Sisters describe their actions as witches and how they are going to mess with Macbeth.

I will drain him dry as hay:
Sleep shall neither night nor day
Hang upon his pent-house lid;
He shall live a man forbid:
Weary se'nnights nine times nine
Shall he dwindle, peak and pine:
Though his bark cannot be lost,
Yet it shall be tempest-tost. (Act 1, Scene 3)

Macbeth has no idea what is going to happen.  He encounters the witches and takes their prophecies very seriously, although Banquo is skeptical.  He also worries about Macbeth’s reaction.  While Banquo is inclined to think the whole thing silly, Macbeth seems highly affected by the witches and their predictions.

Good sir, why do you start; and seem to fear
Things that do sound so fair? I' the name of truth,
Are ye fantastical, or that indeed
Which outwardly ye show? My noble partner
You greet with present grace and great prediction
Of noble having and of royal hope … (Act 1, Scene 3)

The witches make three predictions.  They predict that Macbeth will be Thane of Cawdor and that he will be king.  They also predict that Banquo’s sons will be king.  These predictions have a great effect on Macbeth.  When he finds out that he is not named King Duncan’s heir, Macbeth has a strong reaction.

 The Prince of Cumberland! that is a step
On which I must fall down, or else o'erleap,
For in my way it lies. Stars, hide your fires;
Let not light see my black and deep desires ... (Act 1, Scene 4)

In this aside, he comments that he is ambitious and desires to be king.  This is both foreshadowing and another example of dramatic irony, because the reader knows that Macbeth is going to kill to get what he wants, but Duncan has no idea. He willingly goes to Macbeth’s castle.