In the words of the New York Times' Bob Harris, "Not every coincidence, curiosity, oddity and paradox is an irony, even loosely. And where irony does exist, sophisticated writing counts on the reader to recognize it."
Irony is a complex term and its it is often used in tandem with other literary concepts like sarcasm. As such, it can be difficult to discern the difference between these terms.
For many, the distinguishing the difference between irony and sarcasm has become incredibly difficult to explain. Many maintain that the defining difference between the two terms lies in the idea that Irony can be involuntary, while sarcasm is deliberate.
Irony, generally speaking, can naturally occur in both language and circumstance; one experiences irony when the opposite of an expected situation or idea occurs. In essence, an individual does not need to go out of their way to experience an ironical situation or idea, they can occur naturally.
Sarcasm, for its part, can make use of irony to make an observation or remark about an idea, person or situation. Sarcasm is generally intended to express ridicule or reservation of an expression or idea. Because of this, sarcasm tends to find more broad usage than irony.
The definitions of irony and sarcasm are often conflated, but irony is often confused with another literary term that many people are familiar with--coincidence. Doubtless, there are thousands of examples of this but the most well known instance of this phenomenon can be found in Alanis Morissette's hit song, "Ironic". The song became a chart-topper in mid-1995 and has had a long-term effect on the semantic perception and usage of the word, "ironic".
The song lists and describes a number of experiences as ironic, but if one dissects the song it becomes clear that the use of the word irony was literarily incorrect. The first line of the song, "An old man turned ninety-eight/ He won the lottery and died the next day," does not qualify as an irony. It is decidedly unfortunate that an older man won the lottery and then died, but it would qualify more as bad luck, rather than irony.
In that same vein of logic, rain on a wedding day can certainly be a damper on a celebration but it is a coincidence, not ironic. Another line, "It's meeting the man of my dreams/ And then meeting his beautiful wife," is certainly unfortunate but, again, does not qualify as an irony, considering it is not the opposite of an expected situation.
"Irony deals with opposites; it has nothing to do with coincidence. If two baseball players from the same hometown, on different teams, receive the same uniform number, it is not ironic. It is a coincidence. If Barry Bonds attains lifetime statistics identical to his father's it will not be ironic. It will be a coincidence. Irony is a state of affairs that is the reverse of what was to be expected; a result opposite to and in mockery of the appropriate result."
And, before anyone says it, no, the fact that Morissette's song was not ironic does not make the song ironic.