I wouldnt bother using Irish moss on a stout, what the hell are you gunna see when looking through it anyway (Dark beer)?
Irish Moss (aka IM) is used primarily as an emulsifying agent; Irish moss promotes break material formation and precipitation during the boil and upon cooling of the hot break material. This happens regardless of the color of the wort. While it can be used for purposes of clarity, IM's primary use is in emulsification and coagulation of unwanted lipoproteins in wort (aka hot break material). If fermented, this spooge makes the beer taste cloyingly bitter and astringent, besides making for a cloudy beer. I actually had the misfortune to taste a beer like that at the
Oregon State Fair Amatuer Beer Competition a couple of years ago. It was sooo nasty one judge threw up, another one gagged, and I spat it out on the floor, barely missing the bucket. Needless to say, it was the worst beer I have ever had the displeasure of judging

Hope this clarifies (pun intended) the question a wee bit more. Ttyal, and ilbcnu!
Prost!
Michel