Merriam-Webster defines irony as:
1: a pretense of ignorance and of willingness to learn from another assumed in order to make the other’s false conceptions conspicuous by adroit questioning —called also Socratic irony
2: a) the use of words to express something other than and especially the opposite of the literal meaning
b) a usually humorous or sardonic literary style or form characterized by irony
c) an ironic expression or utterance
3: a) incongruity between the actual result of a sequence of events and the normal or expected result; an event or result marked by such incongruity
b) incongruity between a situation developed in a drama and the accompanying words or actions that is understood by the audience but not by the characters in the play —called also dramatic irony, tragic irony
A simple way of putting it is that irony usually signals a difference between the appearance of things and reality. I loved this example: "Is it ironic that I posted a video about how boring and useless Facebook is on Facebook?" Yes, ironic, definitely, unless you truly made a boring and useless video and put it on Facebook to hide it. If you expected your video to be seen and enjoyed (or to server a purpose) then, yes, that is irony all the way. =))