Heat the olive oil in a large skillet over medium-high heat. Add the lamb chunks and brown them on all sides for about 5 minutes. Transfer the lamb to the slow cooker.
Add the onion, carrots, and celery to the same skillet. Cook for 4 minutes until the onion softens. Stir in the garlic and cook for 1 minute more, then scrape everything into the slow cooker.
Add the potatoes, pearl barley, Guinness, beef broth, tomato paste, thyme, and bay leaves to the slow cooker. Season with salt and pepper.
Stir gently to combine, then cover and cook on low for 7 to 8 hours until the lamb is very tender and the barley is cooked through.
Remove the bay leaves, taste and adjust seasoning if needed, then stir in the chopped parsley just before serving.