Both Cathleen's parents were Irish but she was born in Toronto while her father held the position of assistant professor of mathematics at the University of Toronto. However, when she was two years old the family returned to Ireland when her father was appointed to the chair of Natural Philosophy at Dublin University.
Parehong Cathleen ng mga magulang ay Irish ngunit siya ay ipinanganak sa Toronto ang kanyang ama habang isinasagawa ang posisyon ng assistant professor ng matematika sa University of Toronto. Gayunman, kapag siya ay dalawang taong gulang ang pamilya ibabalik sa Ireland kapag ang kanyang ama ay hihirangin upang angkinin ang upuan ng Natural na Pilosopiya sa Dublin University.