The grain bill for a Guinness-style stout is fairly simple but unless you are brewing all-grain you will have to compromise. You didn't specify but it sounds like you are probably doing extract brews. There is a significant addition of flaked, unmalted barley in Irish stout (10-15%) and it needs to be mashed. There are tons of recipes out there with a quick search of HDT and other sites and Irish stout yeast is offered by both Wyeast and White Labs.
A basic 5 gallon AG:
6.5 lbs UK pale malt
1 lb flaked barley
1 lb roasted barley
1.5 oz 7-8% AA UK hop (Challenger, Norther Brewer, Target, etc) 60 min
Irish stout yeast (Wyeast 1084/ WLP-004)
Mash 60 min @ 153F/67 C
If you don't all-grain brew, sub 5 lbs pale DME for the pale malt and do a mini-mash with 1.5 lbs of pale malt and the lb of flaked barley. The roasted barley could be added to the mini-mash if there's room or steeped on its own.