Excellent question and one that is not simple to answer. I did some leg work and found a few interesting links for you. http://beeradvocate.com/articles/662 http://www.beer-pages.com/stories/guinness-is-good-for-you-2. read more
The hops are ther for bitterness not aroma. Guinness is often produced using a malt and flavour extract from Ireland blended with a local base beer. The beer will be bittered with the best the local brewery has available. read more
Yes, they use hops in Guinness. The amount and TYPE of hops as well as the amount of time they spend in the boil all contribute to the bitterness, flavor and aroma of the end beer. These factors vary from beer to beer and style to style. Some styles of beer contain very few hops. read more
Guinness stout is available in a number of variants and strengths, which include: Guinness Draught, sold in kegs, widget cans, and bottles: 4.1 to 4.3% alcohol by volume (ABV); the Extra Cold is served through a super cooler at 3.5 °C (38.3 °F). read more