Some words are commonly misspelled by writers. Personally, I can't seem to wrap my head around "lieutenant." Some word pairs are mixed up. I can never keep "affect" and "effect" straight, myself. But some words are just used the wrong way...all the time. Ironic, and all its versions, is one of those words.
There's a movie that I love, a romantic comedy with Winona Ryder and Ethan Hawke, named Reality Bites. I love the film because it's true, it does, and Ben Stiller brings a ton of comedic value to the flick. There's a scene in the movie where Ryder's character is put on the spot, and asked to define irony. She makes a few attempts and then cries out that she can't define it, but "I know it when I see it!"
But she probably doesn't. Many people can't define irony, and many writers can't use the word the right way, either. It's a very hard concept to define, but not impossible. And as a writer, it's your job to understand words. Why not start with a really hard one?
By definition, irony means that the outcome is the opposite of what's expected. Now, here's the crux of it: what the hell does that mean?
This is why irony is hard. Let's look at some examples.
"Barbara runs outside every time it rains like this," Sherry explained.
"That's ironic," Brad observed.
Brad's right. Common sense tells us to come indoors when it's raining. The fact that Barbara rushes out into it is ironic, because she's doing the opposite of what's considered to be the norm.
Jessica smiled ironically when she saw the mean portrait of her Becky drew in permanent marker.
We can infer from the above that Becky's portrait of Jessica is less than flattering. This is why her smile is ironic -- we would expect Jessica to look angry, or hurt, instead.
Remember that irony is the opposite of what's expected, and usually it's humorous. When the opposite is sort of funny or amusing, it's clearly ironic.