Irony notes something that is strangely coincidental. An example of irony might be a man afraid of flying who in his first ride in a plane dies in a crash. Ironic detail occurs throughout literature on purpose to make readers think.
Dramatic irony is often cited as a literary device used in plays or drama. Dramatic irony occurs when the audience knows something that a character does not know. It could be that all characters do not know the strange coincidence.
In Othello, throughout the play, readers are privvy to information that one or more characters fail(s) to know. The most significant and on-going piece of dramatic irony occurs as Iago continually plots to execute revenge on Othello for not promoting Iago over Cassio. As Iago makes his different moves to enact this revenge, some characters know of his work while others do not. He plants ideas in Roderigo's head and Cassio's head that ultimately conflict. He employs his wife with the duty of placing the handkerchief Othello had given Desdemona in Cassio's sleeping quarters. Readers know how all of these details happen and we know the innocence of all the characters. However, characters do not figure out the truth until the last act when for many of them, the information comes too late.