Irony is a device that illustrates a meaning opposite to the words written. Often it involves bitterness or mockery. If a principal writes a letter to congratulate a teacher he is firing or a woman writes a love letter to a suitor she intends to reject, the writer is being ironic -- bitterly so. An ironic essay is one that uses opposite meanings to illuminate, for the reader, the author's actual intent. Ironic essays mean the opposite of their content; they mean exactly what they do not say.
Irony by a Master
An excellent example of an ironic essay is Jonathan Swift's "A Modest Proposal." Swift wrote it in 1729 in response to the appalling plight of the Irish poor, who were dying in London streets. He adopts the persona of a cold-hearted analyst who reasons that an Irish infant "is, at a year old, a most delicious, nourishing and wholesome food." His suggested cannibalism is both ironic and hyperbolic. His intention is to bring social injustice to light, a goal his ironic essay achieves far more effectively than a plea for tolerance would have.
Invent an Ironic Persona
One method of writing an essay with irony is to model your work after Swift's and adopt a persona suitable to convey the opposite of your message. If you write an editorial praising an opinion with which you disagree, use Swift's hyperbole and overdo the praise effusively. Readers will know you're not serious. Or you might adopt a "tough-guy" attitude beyond its need. A good example is Frank Sinatra's 1976 letter to Chicago columnist Mike Royko. Royko had complained of Sinatra's added concert security; Sinatra responded with a threatening persona that lampooned his tough-guy image.
Salted with Irony
Essays need not be entirely ironic and written by invented characters. Sometimes touches of irony, sprinkled like table salt to flavor your content, are sufficient. Verbal irony should be used judiciously; it is, after all, sarcasm. "I've had a perfectly wonderful evening," Groucho Marx once said, adding, "But this wasn't it." An ironically twisted phrase like this can be refreshing in an otherwise straightforward essay. Be careful with sarcasm, however, and judicious and tactful with ironic phrases; sometimes they can be inappropriately hurtful.
To Use Irony or Not
To decide whether or not to use irony in your essay, ask yourself your purpose in writing. If you are evaluating familiar subjects or advocating for or against common causes, judicious irony is called for. No one wants to read the same old protest, and a reversal in intent and tone gives readers a surprise and elicits attention. This even works at an essay's end: Mark Twain in his satirical "At the Funeral," after several paragraphs of gently worded etiquette, ends with "Do not bring your dog." That ironic finale is what readers remember.