Ok, You are asking about a sweet stout, but you state that you want the beer to be "very dry." This seems to be an oxymoron, considering most sweet stouts use oats or lactose to increase the perception of body and sweetness. However, I will recommend a yeast for each style of beer.
I prefer 1968 (London ESB) for most styles, including sweet stouts. I think that it is an easy yeast to work with, and gives some nice soft British flavor.
I would recommend 1084 (Irish Ale) for a dry Irish style stout. I have tasted quite a few stouts with this yeast, and I think it gives a nice dry finish.
EDIT: Ok, I can't read