Heat the vegetable oil in a large skillet over medium-high heat. Add the beef cubes in a single layer and brown on all sides. Transfer the beef to the slow cooker.
Add the onion, carrots, celery, and garlic to the same skillet. Cook for 3 to 4 minutes until the vegetables begin to soften. Stir in the tomato paste and cook 1 minute more.
Pour the Guinness into the skillet and scrape up any browned bits. Transfer everything to the slow cooker.
Add the beef broth, pearl barley, bay leaves, thyme, Worcestershire sauce, salt, and pepper to the slow cooker. Stir to combine.
Cover and cook on low for 8 hours until the beef is tender and the barley is cooked through.
Remove the bay leaves. Stir in the fresh parsley just before serving.