Season the lamb shoulder pieces generously with salt and pepper. Heat a large pot over medium-high heat and add 2 tablespoons of vegetable oil. Brown the lamb on all sides for about 5-7 minutes, then set it aside.

In the same pot, add the sliced onion and minced garlic. Sauté for about 3-4 minutes until they are translucent and fragrant.

Return the lamb to the pot, followed by the sliced carrots, diced potatoes, and broth. Add Guinness if using, along with tomato paste, thyme, parsley, and bay leaves. Stir everything until combined.

Bring the mixture to a gentle boil, then reduce heat to low. Cover and let it simmer for 1.5 to 2 hours, or until the lamb is fork-tender.

In a separate pot, boil the peeled and diced potatoes in salted water for about 15-20 minutes until tender. Drain well.

Mash the drained potatoes with milk and butter. Season with salt and pepper to taste.

Once the stew is finished, discard bay leaves and taste for seasoning. Serve hot, ladling into bowls alongside mash.

Garnish with chopped parsley before serving.