Please give an example of dramatic irony from Act 5 in Macbeth? Offer a description of why and how is it dramatic irony.

Another example of dramatic irony--a contradiction between what a character thinks and what the reader or audience knows to be true--occurs in Act V, Scene 5. 

In Scene 4, in the country near Birnam Wood, Malcom and other noblemen with their soldiers move towards the castle of Dunsinane. The general of the English forces, Siward, who is with Malcolm, asks what is ahead of them and he is told that it is "the Wood of...

Another example of dramatic irony--a contradiction between what a character thinks and what the reader or audience knows to be true--occurs in Act V, Scene 5. 

In Scene 4, in the country near Birnam Wood, Malcom and other noblemen with their soldiers move towards the castle of Dunsinane. The general of the English forces, Siward, who is with Malcolm, asks what is ahead of them and he is told that it is "the Wood of Birnam." Malcolm then instructs the soldiers to each cut a branch and use it as camouflage so that their numbers will not be discovered and Macbeth's spies will report inaccurately how many of them there are.

Siward says that he has learned that Macbeth stays inside the castle because his soldiers will only fight if they are forced to do so. Thus, if Malcolm's forces will attack the castle, Macbeth and the others will fight.

In the next scene, Macbeth claims,

Our castle's strength
Will laugh a siege to scorn. Here let them lie
Till famine and the ague eat them up. (5.5.2-4)

When a messenger arrives and tells Macbeth that there is a "moving grove" (5.5.33) approaching the castle, Macbeth recalls the prophecy of the witches. He sounds the alarm and prepares to do battle while taking a fatalistic view: 

Ring the alarum-bell!—Blow, wind! Come, wrack!

At least we’ll die with harness on our back. (5.5.50-51)

This statement is an example of dramatic irony because Macbeth truly believes that Dirnam Wood is moving, but the audience knows that Malcolm has instructed the soldiers to place branches in front of them, which makes it seem as though the forest is moving when it is really not.