Originally Posted by chrisknight
Irish moss is a fining agent. Fining agents all work by making the smaller molecules aggregate into larger particles so they settle out of solution.
To be more exact, it is a kettle coagulant. You can only use it in the boil. If you want a very clear beer, I suggest Irish Moss or Whirlfloc in the boil, then crash cooling and adding gelatin. That always makes a brilliantly clear beer for me.