Add the Guinness, bring to a simmer, and cook for 5 minutes.
Cover and cook on low for 3 hours. Turn the brisket over and cover. Cook for another 3 hours on low. Turn the heat to high and cook for another 3 hours. Open the slow cooker and use two forks to shred the brisket. It should be so tender that it’s just falling apart.
Add back to the slow cooker for 1 hour. Serve on toasted buns.