When you're irritated, you're annoyed. You'd probably be irritated if someone ate the leftover Indian food that you were saving for lunch.
The adjective irritated describes a state that's a little milder than anger. When you're irritated about something, you're impatient or exasperated, the way you feel when someone rubs you the wrong way. Irritated can also mean this quite literally; your wrist might be irritated by the rubbing of your new watch band. The word irritated comes from the Latin irritatus, which means "excite or provoke." Anything that provokes you in an annoying way can make you irritated.