Rinse cornish hens and pat dry. Season liberally with kosher salt and pepper on the outside and inside the cavities.
Mix the apples, cranberries, rosemary, garlic and 1 tablespoon of the olive oil in a small bowl.
Spoon the mixture into the cavities of the cornish hens.
Tie the legs together with kitchen twine and place in a 10 or 12 inch cast iron skillet or appropriate sized baking dish.
Scatter the potatoes around the cornish hens in the skillet.
Drizzle both hens and potatoes with the remaining 2 tablespoons of olive oil.
Cut the butter into small pieces and dot on top of the chicken as well as the potatoes.
Place the skillet in the oven and roast for about 1 hour until the skin has turned golden brown and crispy, the potatoes start to shrivel and the internal temperature of the chicken is about 180 degrees.
Remove from the oven, let rest a few minutes before carving. Serve with accumulated juices from the bottom of the skillet.