Irish Moss is a seaweed derived fining agent used by many brewers to help make a clear beer without the need for a filter. Irish Moss accelerates protein coagulation during the end of the boil which helps prevent chill haze. A majority of the Irish moss plant consists of carrageenan, which is the key ingredient in clarifying beer during the boil.
Typical dosing is 1 teaspoon of Irish Moss for every 5 gallons of wort. Add the desired amount of Irish Moss within the last 20 minutes of the boil for best results.