Heat the oil in a large pot over medium-high heat, add the lamb and brown on each side.
Add the onions and saute until tender, about 5-7 minutes.
Add the garlic and saute until fragrant, about 1 minute.
Sprinkle in the flour and stir.
Add the Guinness and beef stock.
Add the rosemary, thyme, bay leaf, salt and pepper.
Bring to a boil, reduce the heat and simmer until the lamb it fork tender, about 1-2 hours.
Add the potatoes and carrots and some more beef stock to cover.
Bring to a boil, reduce the heat and simmer covered until they are tender, about 20 minutes depending on cut.
Plate and garnish with parsley.
Slow Cooker (RECOMMENDED): Optionally implement steps 1-3, implement step 4, place everything except the parsley in the slow cooker and cook on low for 6-10 hours or high for 2-4 hours before adding the parsley.