Trim and Season the Lamb: Trim away any large pieces of fat from the lamb, then season the pieces generously with kosher salt and black pepper.
Brown the Lamb: Heat the canola oil in a large heavy pot or Dutch oven over medium heat, then brown the lamb for 2–3 minutes per side until well colored. Keep the pieces from touching so they brown properly, working in 2–3 batches if necessary. Transfer the browned lamb to a plate or shallow bowl. Cook the Onions: Add the sliced onions to the same pot with a pinch of salt and pepper, adding a little more oil if the pot looks dry. Cook over medium heat for about 5–10 minutes, stirring occasionally, until the onions have softened and browned. Deglaze the Pot: As the onions cook, scrape the browned bits from the bottom of the pot with a wooden spoon.If they do not loosen easily, add a small amount of the beef broth to help deglaze the pot. Build the Stew: Pour in the beef broth and dark Irish beer, then return the browned lamb and any accumulated juices to the pot.Add the thyme and stir to combine. Simmer the Lamb: Bring the liquid to a boil, then cover the pot and reduce the heat to a simmer.Cook for about 90 minutes, or until the lamb is beginning to break apart easily when prodded with a wooden spoon. The exact time can vary depending on the size and tenderness of the lamb pieces. Add the Carrots and Potatoes: Stir the carrots and potatoes into the stew. Cover and continue cooking for 10–15 minutes. Add the Sweet Potatoes: Stir in the sweet potatoes and cook for another 15–20 minutes, or until all the vegetables are soft and can be pierced easily with a fork.
Thicken the Stew: For a thicker broth, transfer about 1–1½ cups of the cooked potatoes, sweet potatoes, and carrots to a blender.Add 1–2 cups of the stew broth, blend until smooth, then stir the vegetable purée back into the pot. Season and Finish: Taste the stew and adjust the salt and black pepper as needed. Stir in the chopped fresh parsley, if using.
Serve: Ladle the hot stew into bowls and garnish with additional fresh parsley if desired.