When you find something hilarious but you don't want to say " this is hilarious" so you say "I'm hilared." This works because you're basically saying " this is so funny" or something close to that. Can be said in a monotone/ sarcastic voice or in a "I'm dying laughing" voice.
Im watching the new episode of one piece, and Luffy does somethinghilarious. I say "I'm hilared" and laugh.
Someone does something thinking its funny but you don't, "Oh, I'm hilared" sarcasm.