irritant vs pain in the neck

irritant and pain in the neck both are nouns.

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

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

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

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

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