Guinness has pretty much cornered the world market or Irish Stout (so much so that the #1 Guinness-consuming nation in the world is Nigeria). In American bars, Guinness is often the only stout on tap. The beer is practically a synonym for all things Irish. But there are other Irish stouts, often consumed by fiercely loyal homtown drinkers, that are available in better beer stores and pubs.
Murphy's Irish Stout, From Cork, Ireland, is drier and more bitter than Guinness, although just as opaque and creamy. The taste of chocolate is quite noticeable, but without any candy sweetness.
A bartender in Shannon, Ireland told me that, outside of Cork, Murphy's is ordered primarily by tourists. The Corkites (Corkubines?), however, are fiercely loyal to their beer, and rightly so.
Beamish Stout is also brewed in Cork, but has enjoyed less overseas success than Murphy's or the big "G." Expect a more pronounced roasted malt taste, which a lot of people characterize as "Burnt Coffee," and a lighter body.