Toss the beef chunks with flour, salt, and pepper in a bowl until evenly coated.
Heat the oil in a large skillet over medium-high heat. Add the beef and brown on all sides for 4 to 5 minutes.
Transfer the browned beef to the slow cooker. Add the onion, carrots, celery, garlic, and potatoes.
Pour the Guinness into the skillet and scrape up any browned bits, then add this liquid to the slow cooker.
Stir in the beef broth, tomato paste, thyme, bay leaves, and barley.
Cover and cook on low for 8 hours until the beef is fork-tender and the barley is plump.
Remove the bay leaves, taste, and adjust salt and pepper before serving.