On a foggy night in March 1944, Maureen Patricia O’Sullivan parachuted into a field near Limoges, a hotbed of Gestapo activity in Nazi-held France. The 26-year-old Dubliner had been sent undercover by Britain’s Special Operations Executive, or SEO, a secret organisation that supported resistance in enemy territory. O’Sullivan later recalled her mind turning “cold and clear” as she waited for the signal to drop. “After I jumped, it was lovely sailing through the sky on my own,” she told a reporter from the London News Chronicle. “I landed only a third of a kilometre from the Maquis [French resistance fighters],...