"Irk" can be both a verb and a noun.

Do you mean what is the word for "a thing that irks"? There is none really. "Irk" exists only as the verb "to irk" and the adjective "irksome". However, you could say "a pain" or "a pain in the ass" (familiar/vulgar).

Irk or irksome is not used very often; the past participle is irked,." He is troublesome/bothersome." could also be used. "He is annoying" is probably the best way to say it without being vulgar, or like DR Hall said : "He is being a pain"