Heat the olive oil in a large skillet over medium-high heat. Add the lamb pieces and brown on all sides, about 5 minutes total. Transfer the lamb to the slow cooker.
Add the onion, carrots, potatoes, celery, and garlic to the slow cooker.
In a small bowl, whisk together the Guinness, beef broth, tomato paste, thyme, rosemary, salt, pepper, and flour until smooth. Pour this mixture over the meat and vegetables.
Stir everything gently to combine. Cover and cook on low for 8 hours.
Stir the stew once more before serving. Sprinkle with fresh parsley.