Don't worry about it, a lot of people misuse the word "irony," so it is understandable that so many get it wrong simply from how they hear its use. Here's a link detailing what irony actually is in various forms: [link]
Irony can be sarcasm, and in literature a good example of irony is in the Crucible, when Elizabeth Proctor lies for the first time about her husband, John Proctor, being an adulterer. She does this with the intent of saving him, but it ultimately condemns him. Only made more ironic in that John had told the authorities that his wife is an honest woman; she never lies, and the one time he needed her honesty the most she lied.