What is ironic about Jem's certainty that Tom Robinson will be acquitted in Harper Lee's To Kill a Mockingbird?

One definition of irony is "an outcome of events contrary to what was, or might have been, expected" (Random House Dictionary). Used as a literary device, authors can create several types of irony, including verbal irony, situational irony, and dramatic irony. In To Kill a Mockingbird, Harper Lee creates dramatic irony through Jem's naive belief that Tom Robinson will be acquitted.

Dramatic irony is a contrast between a "character's limited understanding of his or situation" as the action develops and the reader's understanding of the same situation (Baker, L., "Dramatic Irony," Critical Concepts, Kansas State University). Being young, naive, and innocent, Jem still has the ability to look at what goes on before him objectively, meaning without bias. From an objective standpoint, the evidence Atticus brought before the court can only reveal one obvious truth: Robinson is innocent. Robinson's innocence is particularly unquestionable considering the fact that Mayella's attacker was said to have bruised the right side of her face, which can only have been accomplished by a left-handed man facing her, whereas Robinson has been crippled in his left arm and hand since childhood, crippled to the point he cannot even keep his left hand placed on the Bible during the oath before taking the witness stand. Therefore, it would have been impossible for Robinson to have attacked Mayella using his left hand, whereas, in contrast, Bob Ewell is, indeed, left-handed.

Yet, what Jem fails to realize that the audience is well aware of is Robinson is not being tried by a jury made of members who will look at the facts of the case objectively; Robinson is instead being tried by a racially prejudiced jury, members of which have judged Robinson to be guilty long before the trial actually took place. Hence, when author Lee has Jem say to Reverend Sykes, "... but don't fret, we've won it ... . Don't see how any jury could convict on what we heard--" (Ch. 21), she is creating irony because she is hinting at the fact that the outcome of Robinson's trial will be contrary to what should logically be expected. In addition, she is creating dramatic irony because Jem's understanding of the outcome of the trial is much more limited than the reader's understanding; at this point in the story, the reader fully understands Robinson's trial is being governed by prejudice.