Preheat the oven to 325 degrees F. Unwrap the brisket and place it fat-side up in a large baking dish. If the brisket came with a spice packet, sprinkle it over the top, then sprinkle on 3 tablespoons pepper and rub it in. Cover the dish with heavy-duty foil and bake for two and a half hours, then uncover and continue baking until the brisket is very, very tender (almost falling apart!) and a fork inserted into the meat goes in easily, about 1 more hour. Remove from the oven and cover loosely with foil to keep warm.
While the brisket is resting, make the balsamic reduction for the cabbage: Stir the balsamic vinegar with the sugar in a small saucepan over medium-high heat and bring to a boil. Reduce the heat to medium low and simmer until the mixture is reduced by two-thirds and thick like syrup, 10 to 12 minutes. Set aside.
Make the cabbage: Increase the oven temperature to 400 degrees. Cut the cabbage into 8 wedges. Working in two batches if necessary, heat the olive oil in a large heavy skillet over medium-high heat. Add the cabbage and sear on both cut sides until the cabbage gets as much color as possible, about 1 minute per side. Use a spatula to transfer the cabbage to a rack placed on a rimmed baking sheet. Season with salt and pepper. Pour the Guinness into the bottom of the pan, then carefully transfer to the oven. Bake the cabbage until tender and deep brown, about 20 minutes.
Slice the corned beef and place it on a platter with the cabbage. Spoon the balsamic reduction all over the cabbage.