The Oxford Companion to Beer definition of
Irish moss
Irish Moss (Chondrus crispus) is a type of edible red seaweed used as kettle (copper) finings. See kettle finings. It grows abundantly along the rocky Atlantic coasts of Europe and North America. The soft body of the plant consists of 50%–60% carrageenan and was used extensively as a kettle finings agent for the past 200 years or more. Its use peaked around 1970 and then sharply declined as other sources of carageenan were developed. Other sources include Euchema spp. and Gigartina spp., which produce carrageenans with slightly different chemical structures to C. crispus. Carrageenans from these different sources are sometimes blended to optimize performance. However, Irish moss and formulated carrageenan blends are also used as auxiliary finings in cask-conditioned beer, often in conjunction with isinglass. Like other types of fining agents, carageenan works through the ability of its electrostatic charge to gather together oppositely charged materials into large clumps, thus allowing them to settle out more quickly from beer or wort.