Only a select few people in the world get irony. The rest think they do. And that's ironic. Possibly.
Irony is the clash of opposites. It is the juxtaposition of what is expected against what happens; what you know and what you think you know.
It is not the same as coincidence. If a ginger person finds that the only drinks left at a party are cans of ginger beer and ginger ale, this is not ironic. If anything, this is the opposite of irony. If there is a convention of ginger hair haters and the only drink available is ginger beer or ginger ale, this may be considered ironic, but it's a long shot.
Nor is it bad luck. If a president is killed by someone accidently discharging their weapon, this is bad luck. If they are killed by someone discharging their weapon in celebration of that president being elected, that is ironic.
If something is appropriate, it is not ironic. If you see a grossly obese person tucking into a kebab, this is not ironic. It is exactly what you expect them to be doing. If you see a fat person deciding to give their heart a break for once and instead of going to KFC, they cross the road to get to the salad bar and are then run down by a truck transporting diet pills which, this is ironic.
If in doubt, just don't use it. It's better to stay quiet, because you're bound to get it wrong. And then your effort to look intelligent will only end up making you look like a bona fide simpleton. Aah, the irony.
Here, irony is perfectly summed up in the simple phrase "I'm not."
The following are cases of the internet lying to you. None of these examples demonstrate irony, only that somebody is capable of being an idiot long enough to caption a poster.