Dramatic irony refers to a situation in which the audience knows something of which the characters in a play are not aware of. The effect is that the words or actions of the characters then take on a different meaning to what is normally understood. An example would be where the audience is aware that a character is being unwittingly led to his doom and his would-be killer tells him that he will take care of him. The irony is obvious.
In Julius Caesar, there are a number of instances in which dramatic irony occurs. We, the audience, know from the outset that Caesar is to be assassinated but he does not. Whatever he or any of the other characters says...
which relates to his imminent murder, becomes ironic, except of course, to the conspirators.
In Act 1, scene 1, when the soothsayer bids Caesar to 'Beware the Ides of March,' he mockingly rejects his warning and calls him 'a dreamer.' The irony lies in the fact that the audience knows that Caesar will be killed on that day and wish that he would heed the warning. Further irony lies in the fact that although the audience knows of his impending doom, there is nothing they can do about it.
Also, in the same scene, the irony is clear when Brutus tells Cassius:
I do lack some partOf that quick spirit that is in Antony.
Without realising it, Brutus is predicting exactly what will happen when Antony is given an opportunity to address the plebeians after he, Brutus, had spoken to them about Caesar's assassination. It is Antony's spirited speech to them that riles them and drives them to rebellion, seeking out all the conspirators.
When Caesar later expresses his reservations about Cassius, Antony tells him:
Fear him not, Caesar; he's not dangerous;He is a noble Roman and well given.
The audience knows that Cassius is Caesar's greatest enemy and was, right at that moment, plotting his murder. Antony's words are, therefore, ironic, not only because of Cassius' plot but also, of all the conspirators, he turns out to be least noble and not 'well given' at all.
In Act 2, scene 1, when Cassius suggests that Antony should also be killed, Brutus advises against it for he does not see any danger in Antony whatsoever. He believes that Antony will be occupied by lesser things and states that Antony is only 'a limb of Caesar', implying that he is powerless without his leader. The dramatic irony lies in the fact that the audience knows Antony's power and how he will use it to punish Caesar's killers.
The fact that Caesar ignores Calpurnia's and the priests' premonitions about the dangers he may encounter is ironic since this is precisely what is to happen. He foolishly ignores their advice, listening rather to Decius Brutus, one of the conspirators, who has come to escort him to the senate-house. The other conspirators accompany him, putting on shows of conviviality when we know that it is all just a deception.
Even more dramatically ironic is the manner in which Caesar addresses his, unbeknownst to him, enemies at the end of Act 2, scene 2:
Good friends, go in, and taste some wine with me;And we, like friends, will straightway go together.
It is also ironic that Caesar refuses to read Artemidorus' petition in Act 3, scene 1, which contains a warning of the plot against him. His vanity becomes his worst enemy, for he says:
What touches us ourself shall be last served.
These words are ironic for it is exactly what happens later, in a physical sense - the daggers which pierce him later in the senate is what he is served last. In the end, we can only conclude that, with all the warnings and predictions that had come before, Caesar himself is as much to blame for his death as the conspirators are since he foolishly and arrogantly refused to believe in his own vulnerability and that, perhaps, is the greatest irony.