DUBLIN — In what seems like a surreal subplot from a Dan Brown novel, a number of irreplaceable religious relics have disappeared from churches across the country, the Irish police and a senior cleric say.

The latest in a series of such thefts involved the removal of the preserved heart of St. Laurence O’Toole, Dublin’s 12th-century patron saint, from the city’s historic Christ Church Cathedral. As there was no sign of forced entry to the cathedral itself, the dean of Christ Church, Dermot Dunne, initially believed the thief had probably hidden in the building when it closed on Friday evening, taken the artifact overnight and simply walked out the next morning.

“Maybe someone stole it to order; it certainly seems plausible,” Dean Dunne said Monday in an interview at Christ Church. “Or maybe a religious fanatic wants the relic and paid somebody to steal it.”...