West Beers : Every Thursday @ The Lamb Brewery, W4 4PH

Oliver, I may have been drunk, but there was no way I was going to believe that G contains fish. You're a beverage racist, that much is clear.

Yup, as others have said:

http://en.wikipedia.org/wiki/Guinness

Guinness stout is made from water, barley, hops, brewer's yeast and is treated with isinglass finings made from fishes' air bladders, although Guinness has claimed that this finings material is unlikely to remain in the finished product.
It isn't vegan as animal products are used in the production process, whether or not Guinness' claim is true.