It can be tough to get the concept of irony across to students. Educator Donald Morton has a great irony resource page with all sorts of activities to help students understand irony and identify the different kinds.
I really like this explanation (from the “review” slide):
Something that is ironic is unexpected.
If it’s unexpected by a character, it’s dramatic.
If it’s unexpected by everyone, it’s situational.
If it’s sarcasm, it’s verbal.