For the Brine: Using a 12-quart stock pot over high heat, bring 2 cups of water to a boil with the kosher salt, sugar, the leftover toasted spices from Irish spice dust, 1 bay leaf, parsley stems and crushed garlic cloves. Stir until sugar and salt have dissolved. Remove from heat and add remaining 6 cups of ice cold water. Submerge the brisket in the liquid, then top with heavy platter to weigh down and keep it completely submerged. Cover and refrigerate for minimum of 3 days or as long as 5 days.
Remove beef from brine. Rinse under cold water to remove any spices that may have stuck to it. Place back into the stock pot and add enough fresh water to cover the beef, about 4 quarts then top it off with the Guinness. Bring to a bubble and then turn it down slightly to a gentle simmer, cover with a lid and cook for 5 hours. During the last 45-60 minutes of cooking, add the cabbage, potatoes, carrots, celery and onions. Cover with a lid and let simmer until the vegetables are tender.
Cut beef against the grain into 1/2-inch slices and arrange on a platter with vegetables alongside. Sprinkle with Irish Dust and chopped parsley, and serve with Pumpernickel bread, Irish soda bread, butter and mustard.