What is "Irish Moss"? I'm trying to reproduce a recipe mentioned in a novel for a beverage, and it mentions that the main character threw in some "irish moss". I assume this isn't literally just moss from Ireland. What is this? Does it have another name I might be able to buy it under?
Irish Moss, Chondrus crispus, is a species of seaweed, rich in carrageenan, an emulsifier and hydrocolloid.

Evidently, you can buy modern extracts as well from brewing supply stores, for use as a kettle fining to help precipitate unwanted proteins.