These are an incredibly rare drop. A friend and I just spent 6 hours killing Anemones in Yhoator Jungle and didnt get a single earring. Got one "Warm Egg" and a few marlboro vines, other than that nothing. He was a lvl 58 PLD/WAR and I am lvl 58 BLM/WHM, and while we were able to take them without a problem, each fight drained both of our mp and required a few minutes of rest.
They don't drop off the ones in Yhoator. You're just wasting your time killing those. It drops off an NM in Eldieme Necropolis under the same name, but this one spawns every 24 (+/-3) hours.
It increases the chance of countering an attack. The passive ability of Counterattack from Monk is needed for this to be of any use. The where abouts of the mob that drops this earring would be well appreciated (grid <pos> of Yhoatar or Necropolis). I would like to have a set myself.
Well, if it says counterattack +1, wouldn't it increase the monk's chance to pull off counter? A recent update has provided many items which could increase subjob abilities as well as the AF gear increasing your main job's abilities.
I seriously doubt this would do anything for those without the counterattack ability.