Heat the oil in a large pot over medium heat. Add the lamb chunks and brown them on all sides. Remove the lamb and set it aside.
Add the sliced onions to the same pot and cook until they soften and turn light brown. Stir in the minced garlic and cook for one more minute.
Sprinkle the flour over the onions and stir for about 30 seconds to coat everything. Pour in the Guinness and scrape the bottom of the pot to loosen any browned bits.
Return the lamb to the pot. Add the beef broth, carrots, potatoes, and rosemary sprigs. Season with salt and pepper.
Bring the mixture to a boil, then reduce the heat to low. Cover and simmer for about 1.5 hours, stirring now and then, until the lamb is tender and the vegetables are soft.
Remove the rosemary sprigs before serving.