Place brisket, including package ingredients and vegetables that are cut into large chunks (the seasoning onion, carrot & celery) into a large soup pot. Add the Guinness and the stock to cover the meat, add water if the meat is not covered. Bring to a boil and skim off any foam that rises to the top, then turn heat down to a simmer. Cook for 45 minutes per pound. Remove the seasoning vegetables and add remaining carrots and potatoes. Simmer about 15 minutes then add cabbage last and simmer for another 10 to 15 minutes.