Hello, Soheil. We often use this term in the same way we use "drives me crazy." Something that "irks" you is annoying to you. It is something that we might express to a friend or colleague when we are frustrated. When we say, "What really irks me is...." we are expressing what really annoys us.

It means 'The thing that irks me about you', so is probably a title? E.g., The thing that irks me about you is your selfishness.