Trim meat of any fat or gristle and cut into 2-inch cubes. Toss beef with 1 T. of the oil. In a small bowl, season the flour with salt, pepper and cayenne. Toss meat with seasoned flour.
Heat remaining 2 T. of oil in a large skillet over high heat. Brown the meat on all sides. Reduce the heat. Add the onions, crushed garlic, tomato paste and 4 T. water to the skillet. Cover and cook gently for 5 minutes.
Transfer the contents of the skillet to a casserole and pour half of the Guinness into the skillet. Bring the Guinness to a boil and stir to dissolved the caramelized meat juices on the pan. Pour over the meat along with the remaining Guinness. Add the carrots, potatoes and thyme. Stir and adjust seasonings.
Cover the casserole and simmer over low heat or in a 300º oven until the meat is tender 2-3 hours.