irritant vs pain in the ass

irritant and pain in the ass both are nouns.

Word	Noun	Adjective	Verb	Adverb
irritant	Yes	No	No	No
pain in the ass	Yes	No	No	No

As nouns, pain in the ass is a hypernym of irritant; that is, pain in the ass is a word with a broader meaning than irritant:

irritant: something that causes irritation and annoyance
pain in the ass: something or someone that causes trouble; a source of unhappiness

Other hypernyms of irritant include annoyance, bother, botheration, infliction, nuisance, pain, pain in the neck.

irritant (noun)	pain in the ass (noun)
something that causes irritation and annoyance	something or someone that causes trouble; a source of unhappiness