Writer's Digest, October 1992, pages 29-30, had an article by William M. Ross called The Power of Plot Irony. The subtitle suggests that "this technique can instill your story plots with drama, depth – and unexpected consequences."
He starts out by reminding us that the traditional good plot "has a problem/resolution structure: a character faces a problem, struggles with it over the course of the story, and then solves it at the end with a striking action." Good definition, but how do you make the struggle, the story, compelling? Well, plot irony "can give your story the kind of unexpected plot turns that keep readers turning pages."
Next, he describes a story by Max Brand called "Wine on the Desert" as an example plot irony. An outlaw wanted for murder, fleeing the sheriff, visits his old friend Tony who has a vineyard irrigated by vats of rainwater. Tony is friendly, but the outlaw shoots holes in the vats to keep the sheriff from having the water he needs to follow him. Then he orders Tony to fill his canteen with water. The outlaw grabs the canteen and heads into the desert.
So far, a man on the run, friendship betrayed, rugged setting, lots of action. But… Where's the irony? Well, as the outlaw finds out deep in the desert, Tony filled the canteen with wine, not water. That's the irony.
Now, he explains that plot irony is not achieved by a single incident, but by a pattern of incidents in a specific configuration. You need three interlocking events. First, someone misperceives a situation. Second, he acts on the basis of this misperception. Third, as a result of the action, he experiences unanticipated consequences, either positive or negative.
Since this is usually revealed at the end of the story, often say they like the final twist. What they really mean is they like the ironic pattern which is revealed at the end.
In the story about wine, the outlaw mistakenly believes that his friend Tony filled his canteen with water. He acts on the misperception by running further into the desert. He suffers unanticipated consequences at the end. Separately, these incidents are not ironic, but together they are.
Next, Ross takes a look at a story by O. Henry, The Furnished Room, which he says fails! The misperception is not really wrong, which means that the ending doesn't quite work. So, make sure that your character has a serious misperception. Next, O. Henry doesn't really tie the three incidents together. Specifically, there's really no unintended consequences. So make sure that your protagonist's misperception causes the action and the unanticipated consequences. Finally, O. Henry didn't characterize his characters very well. So even if you're going to use plot irony, don't neglect characterization.
Next, Ross looks at Jack Finney's Of Missing Persons and assures us that in this story, the characterization supports the well constructed irony. The ironic events tie together. The misperception is a major one. And the character traits match the perceptions and the actions.
Finally, Ross walks us through his own construction of a play using plot irony. He started with two brothers, and a situation. Jeopardy, one of the brothers is very invested in his business. But, he starts embezzling money. The other brother threatens him. The first brother sees his other brother as a threat. (A mistaken perception?) So, he lures him out and shoots him. (The action due to the misperception!). Now, a detective lays out all of his evidence, and the first brother pulls out the gun again, explains what happened, and tries to kill the detective. However, the detective had already emptied the bullets from the gun. Then the detective reveals that the dead brother had collected money to help out his brother! That's the irony, the revelation of the misperception.
So, now it's your turn. Have a character misperceive a situation. Then have them act on that misperception. And finally, let them experience unanticipated consequences of the action, either positive or negative. To make it plausible, make sure your characters have the traits that they need so that the perceptions and the actions feel real.
There you go. A dash of irony for your stories!