Just examine Act 2 for an example of dramatic irony in "Macbeth". The act is filled with irony. In the first scene, Macbeth sees the floating dagger and knows the irony of it; that it represents the guilt he already feels for the act he hasn't yet committed. Lady Macbeth boasts in the opening of scene 2 that the alcohol that made the guards drunk has made her bold and strong. She then chastises her husband for being so upset about the act of killing Duncan. She tells him that a little water cleanses them as it cleans the blood off their hands, yet later, in Act 5, we see that she has gone insane and continually tries to wash imaginary blood from her hands. Her strength has turned to weakness and has driven her mad. At the end of that scene, Macbeth wishes that the knocking at his palace gate could awaken Duncan, but of course, he's just killed Duncan. In the next scene, the porter imagines he is the porter at hell's gate. Considering he is the porter at the gate of the castle where regicide has just been committed, he is somewhat the porter of hell's gate. At the end of that scene, Donalbain remarks that where they are, there are "...daggers in men's smiles." That's true because Macbeth and his wife pretend to be their friends, but they are the murderers. Scene 4 is ironic in the old man's explanation of the weird and unnatural events that took place the night of Duncan's murder. The weird happenings mirror the unnatural act of killing a king.
In Macbeth, Shakespeare undercuts much of the dramatic irony in the play through the use of equivocal language by the witches and the Porter and by combining the roles of hero and villain into one character, Macbeth.
In this way, we as audience do no know much more than Macbeth regarding the validity of the prophecies. We know Macbeth is a villain, but we don't know for sure who is good and, therefore, his nemesis. The play is so devoid of goodness that we don't trust anyone (the witches, Malcolm). We even have doubts about Macduff, who leaves his family to slaughter.
Granted, we are not as hopeful as Macbeth regarding the prophecies, and we know this is a tragedy and that Macbeth will not survive, but--in a way--we still root for him to defy the odds and fate. We want a lone individual with courage to succeed. This drains much of the dramatic irony out of the play.
About the only examples of dramatic irony regard murder and madness. The play is quite the opposite of a detective story, for:
1) We know the Macbeths plan to kill Duncan
2) We know Macbeth plans to kill Banquo
3) We know Macbeth will kill Macduff's family
4) We see signs of Lady Macbeth's madness
5) We know Hecate and the Witches have it in for Macbeth
Still, there is detective work to be done regarding the second set of prophecies. But, we cannot expect to know that riddles that will play out in Act V: that Macduff's mother died before delivery; that the soldiers would use wood as a disguise; that Macbeth will be beheaded. We are in the same position as Macbeth regarding these events.