Heat the olive oil in a large Dutch oven over medium-high heat. Add the lamb cubes and brown them on all sides for about 6 minutes.
Add the onion, garlic, and carrots to the pot. Cook for 4 minutes until the onion softens.
Pour in the diced tomatoes, Guinness stout, beef broth, oregano, salt, and black pepper. Stir to combine.
Bring the mixture to a boil, then reduce heat to low. Cover and simmer for 1 hour 15 minutes until the lamb is tender.
Stir in the Kalamata olives and cook uncovered for 5 more minutes.
Ladle the stew into bowls and top each serving with crumbled feta and fresh parsley.