Can you explain an example of irony found in act 3, scene 3?

Shakespeare loves irony, and there are several examples of it in this scene.
First, for example, Emilia notes that the trouble between Othello and Cassio

grieves [her] husband / As if the cause were his.

Emilia at this point doesn't know that Iago is, in fact, the cause of the trouble between Cassio and Othello, nor does she realize that the trouble is a source of pleasure, not grief, to her husband. This is an example of dramatic irony, which occurs when characters are not aware of information that the audience already possesses.
Another example of dramatic irony occurs when Desdemona refers to Iago as “an honest fellow." We as audience know, of course, that Iago is anything but an honest person. He is evil and devious and plots to destroy Desdemona.
Desdemona's heartfelt desire to help Cassio is also laced with irony, as Iago plans to use this innocent desire to make her look as if she is cheating on Othello. If she were a meaner and more hard-hearted person who refused to help a friend in need, she would, ironically, end up appearing more innocent to her husband.