Heat the olive oil in a large skillet over medium-high heat. Add the beef cubes and brown on all sides for about 5 minutes. Transfer the beef to the slow cooker.
Add the onion, carrots, celery, and garlic to the same skillet. Cook for 3 minutes, stirring often, then transfer everything to the slow cooker.
Stir in the pearl barley, Guinness stout, beef broth, tomato paste, thyme, and bay leaves. Season with salt and pepper.
Cover and cook on low for 8 hours or until the beef is tender and the barley is cooked through.
Remove the bay leaves before serving. Taste and adjust seasoning if needed.