Simple: 70% pale ale malt, 20% flaked barley, 10% roasted barley. One bittering hops addition, IBU ~50. Ferment with a very attenuative yeast, in the 80% range. Shoot for an ABV no more than 4%.
True Guinness is said to be blended with a portion of soured stale beer. Try that at your own risk.
Cheers! (Or should I say slainte?)
Bob