Heat the vegetable oil in a large Dutch oven over medium-high heat. Add the lamb cubes and brown them on all sides, working in batches if needed to avoid crowding the pan.
Lower the heat to medium and add the diced onion. Cook for 3 to 4 minutes until the onion starts to soften.
Stir in the sliced carrots and cook for another 2 minutes.
Pour in the Guinness and let it bubble for 1 minute while scraping the bottom of the pot.
Add the lamb stock, pearl barley, bay leaves, and dried thyme. Season with salt and black pepper. Bring the mixture to a simmer.
Cover the pot and cook on low heat for 1 hour, stirring occasionally.
Add the potato chunks and continue simmering, uncovered, for 45 minutes until the lamb is tender and the potatoes are cooked through.
Remove the bay leaves. Taste and adjust salt and pepper if needed. Sprinkle with chopped fresh parsley before serving.