What is an example of dramatic irony in Act 2, Scene 3, lines 100-112 of Macbeth?

First, to understand this passage, it's helpful to understand the definition of dramatic irony. Dramatic irony occurs when both the audience and one or more of the characters in a play know something that the other characters on stage do not. In this scene, Macbeth is present when the body of King Duncan is discovered. Macduff has come to see the king in the morning, and asks if he is awake yet. Macbeth says:

MACBETH:...

First, to understand this passage, it's helpful to understand the definition of dramatic irony. Dramatic irony occurs when both the audience and one or more of the characters in a play know something that the other characters on stage do not. In this scene, Macbeth is present when the body of King Duncan is discovered. Macduff has come to see the king in the morning, and asks if he is awake yet. Macbeth says:

MACBETH: I'll bring you to him.

MACDUFF: I know this is a joyful trouble to you, but yet 'tis one.

MACBETH: The labor we delight in physics pain. This is the door.

MacDuff goes in and soon comes rushing back out, saying that Duncan has been murdered. Shortly Lady Macbeth arrives, asking what the commotion is:

LADY MACBETH: What's the business, That such a hideous trumpet calls to parley The sleepers of the house? speak, speak!

Now begins the passage cited in the question. In response to questioning by Malcolm and Donalbain as to what is amiss, Macbeth replies:

MACBETH: You are, and do not know't: the spring, the head, the fountain of your blood is stopt--the very source of it is stopt.

MACDUFF: Your royal father's murder'd.

In this passage, the dramatic irony comes from the fact that the audience knows who murdered King Duncan. Both Lady Macbeth and Macbeth know intimately well who murdered King Duncan, but the other characters on stage do not. Macbeth and his wife knew the King was dead, and by whose hand the murder was committed, before Macduff ever showed up. Yet Macbeth and his wife try to pretend that they are just as shocked and horrified as the other characters in order to maintain the appearance of their innocence.