Sold by the ounce Irish moss is not a moss at all, but a seaweed. It is used in beer brewing, particularly in professional and competition beers. It does not alter the taste, but does make the beer less cloudy, making it more attractive to some discerning drinkers. The key is to add it at the right time in the brewing process. If you add Irish moss too early or too late, it may work less effectively or not at all.