Remove brisket from the fridge 30 minutes before you’re ready to cook. Combine Salt, pepper, dried thyme, rosemary, garlic powder, and onion powder in a small bowl or ramekin. Rub olive oil on the brisket, then season generously. Let the rest and come to room temp for 30 minutes.
Slice the onion in half. Dice one half of the onion, and thinly slice the other half.
Heat a saucepan over medium heat. Add olive oil and swirl to coat. Next, add the diced onions, shallot, and garlic cloves to a saucepan over medium heat. Cook for 5 minutes, then add cranberry, Worcestershire, and Guinness. Stir to combine and cook for 10 minutes.
Preheat the oven to 250 degrees F.
Pour half the saucepan into the roasting pan. Add sliced onions and place brisket fat side down on top. Pour remaining sauce over the top of the brisket.
Surround the sides of the brisket with potatoes, dried cranberries, and fresh thyme and rosemary sprigs. Cover with foil.
Place onto the oven on the center rack and bake for 4 hours.
Slice brisket and serve with potatoes and sauce.