Preheat the oven to 325°F and set a rack in the lower middle position.
Dry the lamb with a paper towel and season with the salt and pepper.
In a large Dutch oven, heat three tablespoons of olive oil or bacon grease ( over medium-high heat. Brown the meat in batches for about 5 minutes per batch. Sear the meat until it develops a nice brown crust but no need to cook the meat through.
Transfer the meat to a warm plate and set aside.
Add the onions, garlic, and dried rosemary; cook, stirring with a wooden spoon and scraping the brown bits from thwe bottom of the pan, for about 5 minutes.
Stir in tomato paste and cook for another minute.
Sprinkle with the flour and stir until the flour is dissolved for about two minutes.
Add the Guinness, beef broth, water, and bay leaf. Stir and loosen any brown bits from the bottom of the pan and bring to a boil.
Add the seared lamb and all the juices back to the Dutch oven.
Cover the pot with a lid, place into preheated oven, and braise for 2 hours.
Remove the pot from the oven and add the carrots and potatoes. Cover and place back in the oven for another 45 minutes to one hour more, or until the vegetables are softened, the broth is thickened, and the meat is tender.
Remove the bay leaf and discard, then taste and adjust seasoning add more salt and pepper if necessary.
This stew always tastes better the next day refrigerating overnight allows the flavors to melt together.
Garnish with fresh parsley.
Enjoy with your favorite crusty bread.