Preheat oven to 425°.
Remove white papery skin from garlic head (do not peel or separate cloves). Place potatoes and garlic in a saucepan. Cover with water; bring to a boil. Drain immediately; pat dry.
Combine the chopped rosemary, oil, salt, and pepper in a large bowl; add potatoes, tossing to coat. Arrange potatoes and garlic on a jelly-roll pan. Bake at 425° for 30 minutes or until tender, stirring occasionally. Separate the garlic head into cloves, and discard skins; serve with potatoes. Garnish with rosemary sprigs, if desired.