Possible Clues:

• Two of the Kings of Ireland, one upset by heartless taunt that's a thorn in someone's side
• Inflammation-causing substance such as pollen
• It's annoying
• Cause of itch
• Irish girl joins National Trust — a cause of annoyance
• Woman educated in Irish books causes trouble
• Taxman getting it in angry tirade is a nuisance
• Grain of sand, to a pearl oyster
• Nuisance
• Smoke in one's eyes, say