Greetings Pathfinder - I don't use Irish Moss any longer, but I recall using about 1-2 teaspoons in a 5-gallon batch.
Update:
I use a product called Super Moss. It's a fine powder made by 5-Star. I add about a 1/4 tsp. 10 min before the end of the boil. I no longer use Irish moss because I have a plate cooler and I don't want any issues with clogging. I also add a product called Clarity Ferm, a liquid by White Labs, directly into my fermenter. The combination of these two products makes my beer crystal clear.