For nine seasons, Mary McDonough was part of one of the most beloved families in television history. Just ten years-old when she was cast as the pretty, wholesome middle child erin, mary grew up on the set of, alternatel...For nine seasons, Mary McDonough was part of one of the most beloved families in television history. Just ten years-old when she was cast as the pretty, wholesome middle child erin, mary grew up on the set of, alternately embracing and rebelling against her good-girl onscreen persona.