Dramatic irony exists when the audience or reader knows something is going to happen and the characters involved do not. In regards to William Shakespeare's Macbeth, many instances of dramatic irony exist.
Duncan's arrival at Inverness (Macbeth's castle): "This castle has a pleasant seat" (I,vi, 1).
When Duncan arrives at Macbeth's castle, he has no clue that Macbeth has planned to murder him. Duncan is openly welcomed by both Macbeth and his wife, compounding the irony.
Macbeth visits the witches, again: "And damned all those that trust them" (IV, i, 144).
When Macbeth damns the witches, the audience knows that he is damning himself (he does not recognize this). This also foreshadows his death (traditional of the tragic hero due to his hamartia (tragic flaw).
Enter Macbeth: "There’s no art / To find the mind’s construction in the face. / He was a gentleman on whom I built / An absolute trust" (I, iv, 12-14).
Here, Duncan is saying that he is no good at judging a book by its cover. He did this with the old Thane of Cawdor and was betrayed. Here, as soon as he states this, Macbeth enters. Readers know that Macbeth will be the next to betray the king.