Pat the lamb dry and season it with salt and pepper. Heat the olive oil in a large pot over medium-high heat. Brown the lamb in batches until all sides are seared, then remove it to a plate.
Lower the heat to medium. Add the onions and carrots to the pot and cook until the onions soften. Stir in the garlic and cook for one minute more.
Sprinkle the flour over the vegetables and stir for one minute to coat them. Pour in the Guinness, scraping the bottom of the pot to release any browned bits.
Return the lamb to the pot along with the beef broth, thyme, and bay leaves. Bring the mixture to a simmer, then cover and cook for 45 minutes.
Add the potatoes and continue simmering, uncovered, for 25 to 30 minutes until the potatoes are tender and the lamb is soft. Remove the bay leaves before serving. Garnish each bowl with chopped parsley.