This is probably the primary reason I got into homebrewing. With a case of Guinness, in my neck of the woods, costing around 35 bucks, it doesn't take a genius to fiqure out that it is cheaper to make your own. The original recipe is for an all-grain brewer, but I have listed the substitutions for the extract brewer,
7 pounds, Crushed pale malt
2 pounds, Flaked barley
1 pound, crushed roast barley
1 ounce, bullion hops
3 ounces, northern brewer hops
1 tsp. CaCO3 (if you are in a soft water area)
yeast starter made from a bottle of Guinness or a liquid yeast
Extract brewers: Substitute 2 cans of a light extract for the 7 pounds of pale malt. Also, if you don't want to make a yeast starter use Whitelabs Irish Ale yeast or Wyeast Irish Ale yeast.
I generally boil at 60 minutes, 30 minutes and 15 minutes and add my hops in at those intervals. At the 15 minute mark, I also use Irish moss to help settle the solids..