Place the beef stew meat, sliced carrots, diced onion, and minced garlic into the slow cooker.
Add the pearl barley, Guinness, beef broth, tomato paste, dried thyme, bay leaves, salt, and pepper.
Stir everything together until the tomato paste is mostly dissolved.
Cover and cook on low for 8 hours until the beef is tender and the barley is cooked through.
Remove the bay leaves before serving.