On a cold December night in 2000, 28-year-old Sandra Collins left her aunt’s home in Killala, County Mayo, saying she was going for chips — and vanished. Four days later, her fleece was found folded on the pier, the sausages she bought still in the pocket. No trace of her has ever been found. Twenty-five years later, her family is still searching for answers in one of Ireland’s most haunting mysteries.

A haunting journey into Ireland’s most enduring disappearance: the young American who vanished in plain sight and left a country...