SCHULL, Ireland—She was a chic Parisian film producer with a holiday home in one of the most beautiful and remote corners of Europe: an isolated, windswept peninsula at the southernmost tip of Ireland.

Sophie Toscan du Plantier could have been a heroine out of a Daphne du Maurier novel. Instead she ended up the victim in one of the worst horror stories in this country’s recent history. Her brutal murder—she was bludgeoned while fleeing her desolate farmhouse onto the vast moors in the middle of the night in her pajamas—remains unsolved 18 years later.

Sophie’s husband, Daniel Toscan du Plantier, a prominent French film producer who worked with Fellini, Bergman and Kurosawa, told Le Figaro after the murder that “there’s a devil somewhere in the hills of southern Ireland.”