Toss the beef with the flour, salt, and pepper in a bowl until evenly coated.
Heat the oil in a large skillet over medium-high heat. Brown the beef in batches, then transfer it to the slow cooker.
Add the onion, celery, carrots, and garlic to the same skillet. Cook for 3 minutes, stirring to loosen any browned bits.
Pour the Guinness into the skillet and simmer for 2 minutes. Scrape the bottom of the pan, then transfer everything to the slow cooker.
Add the beef broth, potatoes, tomato paste, thyme, and bay leaves to the slow cooker. Stir to combine.
Cover and cook on low for 8 hours until the beef is fork-tender and the gravy has thickened.
Remove the bay leaves before serving.