In conversation a person's attitude toward what he is talking about is partly revealed through his tone of voice.  The most ordinary expression or wrld ("Really!") can be used to communicate anything from wonder to sarcasm.  A writer, unable to rely on inflection, must find verbal means of establishing a tone that will convey his attitude toward his subject.  This tone lets the reader know how he is meant to regard a particular character...
    An ironic tone depends upon a contrast between what the writer's words seem to say and what they really mean.  To grasp the true meaning we must read between the lines.  Verbal irony can take various forms, some easier to detect than others.

Reaske, Christopher R.  Mirrors: An Introduction to Literature. 3rd ed.  New York: Harper and Row, 1988, 197.
 

Irony results when there is a difference between what appears to be happening and what is actually happening.  For example, when a character or reader expects or assumes one thing and the opposite is true, the writer has created irony.

Literature and Language: English and World Literature.  Evanston, Illinois: McDougal, Little & Co., 1992, 652.

Irony is created when a writer says one thing but really means something else.

Literature and Language: English and World Literature.  Evanston, Illinois: McDougal, Little & Co., 1992, 901.
 

Situational irony occurs when a reader or character expects one thing to happen, but something entirely different happens. . . .  Writers use situational irony to make their stories interesting or humorous, and sometimes to force their readers to reexamine their own thoughts and values.

Literature and Language: English and World Literature.  Evanston, Illinois: McDougal, Little & Co., 1992, 161.