The dramatic irony of this Act not only emphasizes the fact that Emelia is describing her own husband that she does not know has masterminded the jealous plot that has caused the death of her maiden, Desdemona, but it also points out that Emelia has unknowingly participated in the deadly plot by stealing the prized handkerchief from Desdemona and giving it to Iago. This handkerchief is the catalyst that sets up the proof for Othello that Desdemona has been unfaithful with Cassio. Ironically, the audience (readers) know all of this, but poor Emelia is clueless.
This is dramatic irony because Emilia doesn't know that it is her husband, Iago, who is the villain she speaks of. She speaks harshly of a man who would do such a thing, calling him an "insinuating rogue" and a "cogging, cozening slave." We know this perfectly describes Iago (so perfectly, in fact, that some would question whether Emilia already knows it is Iago). As she says this, Iago is right beside her, saying "There is no such man; it is impossible," also ironic, for we know he is such a man.