Heat the olive oil in a large Dutch oven over medium heat. Add the lamb chunks and brown them on all sides for about 6 minutes. Remove the lamb and set it aside.
Add the chopped onion to the same pot and cook for 3 minutes until it softens. Stir in the garlic and cook for 1 minute more.
Return the lamb to the pot. Add the sliced carrots, cubed potatoes, and tomato paste. Stir everything together for 2 minutes.
Pour in the Guinness stout and beef broth. Add the thyme sprigs and bay leaves. Season with salt and pepper.
Bring the mixture to a boil, then reduce the heat to low. Cover and simmer for 1 hour and 30 minutes, stirring occasionally, until the lamb is tender and the vegetables are soft.
Remove the thyme sprigs and bay leaves. Taste and adjust seasoning if needed. Sprinkle with chopped parsley before serving.