In stable irony there is a constant perspective from which to perceive the underlying meaning; whereas in unstable irony there is no perspective that is not itself undercut ironically. (See allegory, satire, point of view .)
Dramatic Irony is a situation in which the reader or audience knows more about the immediate circumstances or future events of a story than a character within it; thus the audience is able to see a discrepancy between characters' perceptions and the reality they face. Characters' beliefs become ironic because they are very different or opposite from the reality of their immediate situation, and their intentions are likewise different from the outcome their actions will have.
Audiences familiar--like the original Greek spectators--with the legend of Oedipus know that the hero of Sophocles' tragedy is guilty of the evil he seeks to punish, and they can therefore fully appreciate Oedipus' blindness as he self-righteously hunts down the murderer of Laius--not only blind that he is the murderer, but that Laius is his father and that he is presently married to his own mother.
Similarly, Othello's hatred of Desdemona for cuckolding him is more horrible and tragic because the audience knows he is deceived by Iago and can watch every step of his error. Dramatic irony can produce comic effects when the ignorance of characters merely makes them appear ridiculous, or when the unintended results of their actions are humorous.
Structural irony occurs when a double level of meaning is continued throughout a work by means of some inherent feature such as a hero, narrator, or persona who is either naive or fallible (a participant in the story whose judgment is impaired by prejudice, personal interests or limited knowledge).
In Swift's Gulliver's Travel's (1726), the narrator who recounts his own travels is both naive and fallible: he credulously idealizes some of the peoples he encounters despite their follies; and his judgments are biased by conservative morality and personal pride. Narrow conventional morality also biases Nelly Dean, the narrator of Emily Bronte's Wuthering Heights (1847); and in Shakespeare's As You Like It, the continuing disguise of Rosalind as the young man Ganymede leads to multiple levels of dramatic and structural irony.
Verbal irony occurs when the words of a character or narrator have an implicit meaning as well as an ostensible one. The surface meaning may be false, or it may be a level of meaning that is just very different from the underlying one (which is usually more significant). One can guess when words should not be taken at face value by the context in which they occur--which includes the speaker's character, the situation, particular word associations, and a common ground of assumptions shared by the speaker and the reader.
Hamlet's character as a brooding intellectual produces language that is loaded with ironic witticisms, as when he tells his hated stepfather "I am too much in the sun" (punning on "son").