What is the difference between those adjectives? I've understood that facetiously is used when you're saying something that you don't mean- you're joking, but what differentiates it from (humorous) irony? When you're ironic you have to actually mean something else than what you say, is that the case with being facetious as well? Or is that just saying something you do not mean?
Strictly speaking, these are two different words with two entirely different meanings. In common speech, there are times when they are used interchangeably.
Facetious is probably best summed up by Collins as:
If you say that someone is being facetious, you are criticizing them because they are making humorous remarks or saying things that they do not mean in a situation where they ought to be serious.
People would not self-apply the label of facetious in situations where others accuse them of being so. People who are accused of being facetious probably think they are being witty, which is in fact the original meaning of the word.
1590s, from French facétieux (16c.), from facétie "a joke" (15c.), from Latin facetiae "jests, witticisms" (singular facetia), from facetus "witty, elegant, fine, courteous," which is of unknown origin, perhaps related to facis "torch."
Formerly often in a good sense, "witty, amusing," but later implying a desire to be amusing that is often intrusive or ill-timed.
Humor doesn't always land. When it doesn't, it is viewed as unwelcome. Facetious has come to be a pejorative, used similarly to flippant (meaning 1).
However, there are times when people will describe themselves as being facetious. This is when they employ verbal irony that could be mistaken for being earnest and they want to explain they are joking around, not being serious.
Here's an example from Arkansas Online:
"Oh, she's just doin' what you do to get elected," Bubba said. "Clinton did the same thing."
Yeah, I remember Clinton standing in the Fort Chaffee door and railing against the evil federal government when Jimmy Carter sent Cuban refugees there.
I was being facetious. Clinton couldn't stop it, wouldn't try and didn't think he should. And he lost.
I've italicized the portion of the text where verbal irony is taking place.
Webster's defines irony (in the sense we are using, often called verbal irony) as:
1 : the use of words that mean the opposite of what you really think especially in order to be funny
a writer known for her clever use of irony
"What a beautiful view," he said, his voice dripping with irony, as he looked out the window at the alley.
She described her vacation with heavy irony as "an educational experience."
In common use, "I was being ironic" and "I was being facetious" are used interchangeably in this circumstance, with the later being slightly more common according to Ngram. But neither of these is used as often as "I was being sarcastic," which is most often used to mean ironic in casual speech, despite the fact not all sarcasm employs irony, and not all irony is sarcastic.
In most situations, ironic and facetious are not interchangeable. You could describe a satirical film or book as using irony, but you would probably not consider it facetious as the humor in the work is intentional and deliberately placed. Likewise, you would not chastise a person as being ironic who made an out-of-place joke in bad taste. You'd chide them for being facetious.
To be facetious is to be inappropriately, often offensively, humorous but not to say the opposite of what is meant.
For example if someone drops a tray of glasses and someone else says "You didn't want to do that, did you?" that would be facetious, particularly if it was said in a mocking tone of voice, because the person genuinely did not want to drop the tray. If the other person said "Nice display of skill and control." that would be irony because dropping the tray is a failure of control and possibly a demonstration of a lack of skill.
It is possible to be facetiously ironic but that is a different matter.
Facetiousness is almost always obviously offensive but irony can be so subtle that target of the humour is unaware that they are being lampooned, particularly if they have an over-inflated sense of their own importance.
I apologise for the poor quality of the humour in the examples but my comic scriptwriting skills are not of the best.