Irish moss

Irish Moss is a traditional Jamaican beverage made from seaweed extract, whey, nutmeg, tumeric, and other "secret" ingredients. In its most potent form, served chilled and mixed with rum--that Caribbean social lubricant--Irish Moss is said by the locals to be good for de back, i.e. promoting masculinity and--by extension--sensational sex.

It is and it does.

It's also legal.

Irish moss is the common name for a variety of red seaweed. In addition to the uses mentioned in the above write-ups, one species, Chondus crispus is used in the art of homebrewing. Added to the wort in the last fifteen minutes of the boil, the Irish moss bonds to long protein molecules, effectively removing them from solution. These protein molecules are necessary in small quantities as nutrients for the yeast, but in larger quantities, this can produce haze in the finished beer.

Irish moss does not effect the flavor of the beer at all. If you forget it, don't worry, clarity is an aesthetic quality of beer, and the real point is to drink it.