In Act V scene 5 of Macbeth, Macbeth sees his plans and schemes unraveling. While the witches' prophecies leave him feeling confident (no man born of woman can hurt him, and he will be safe until Birnam Wood comes to Dunsinane Castle.)
However, as the scene begins he learns that Lady Macbeth has committed suicide; he brushes this news off telling Seyton she would have died eventually anyway. The news that does trouble him is when the messenger claims that he saw Burnam Wood moving towards him. (Imagine telling your king that you saw a forest walking towards his castle. Would you not worry you were seeing things?) Macbeth claims that the messenger must be lying, and threatens to hang him:
If thou speak’st false, Upon the next tree shall thou hang alive Till famine cling thee. If thy speech be sooth, I care not if thou dost for me as much. I pull in resolution and begin To doubt th' equivocation of the fiend That lies like truth. “Fear not, till Birnam wood Do come to Dunsinane”; and now a wood Comes toward Dunsinane.—Arm, arm, and out!— If this which he avouches does appear, There is nor flying hence nor tarrying here. I 'gin to be aweary of the sun, And wish th' estate o' th' world were now undone.— Ring the alarum-bell!—Blow, wind! Come, wrack! At least we’ll die with harness on our back.
Here, Macbeth is beginning to doubt what the witches (Devil) has told him and knows that he must go into battle. He tells the messenger to get ready for battle- if what the messenger says is true; they're going to have to fight and must be ready to fight to the death. He even debates that he may be tired of living and "wish th' estate o' th' world were now undone" or that the world would turn into chaos.
But how does he use dramatic irony as a dramtic device to present Macebth's decision to kill Duncan?