A nark is a snitch, someone who grasses you up to the teacher, the bouncer, the landlord, whoever. As a verb, to nark (or nark on) someone is to tell on them. And if something narks you, it’s doing your head in, properly irritating. Old-school British, still pops up in everyday moaning.
"We had a tiny pre-drinks and somehow the landlord knew in ten minutes. Someone’s narked on us. Bet it was Dave, Mr “I love a quiet night in”, the absolute grass"