Guinness Draught, the kind found in widget cans or bottles, is an Irish dry stout. Guinness has a sharper roast character and more hop bitterness than Murphy’s. The key to making a great clone is using roasted unmalted barley (or black barley) with a color rating around 500 °L.
Headspace can cause oxidation, especially in secondary fermenters. Anything that is done to eliminate air helps minimize oxidation. And adding de-aerated water can reduce the odds of oxidation, but the process deserves