King Edward VII, Queen Alexandra, and Princess Victoria were due to visit on July 10, to make an appearance at the exhibition and perform some various royal duties. The political relationship between Ireland and Great Britain was fraught, with a rising tide of Irish nationalism competing with unionists who wanted to remain loyal to the Crown. There had already been debate about how Irish—or British—the International Exhibition should be. (There were separate pavilions for Ireland and Great Britain; the Irish War of Independence would erupt just over a decade later.) On top of that, the king’s nephew, Kaiser Wilhelm II of Prussia, had just months before endured a massive political scandal. King Edward was sensitive to controversy. He needed this visit to go smoothly.
It did not go smoothly.
Four days before the king was due to arrive in Dublin, the jewels went missing. The story of this theft would eventually involve a sex scandal, conspiracies that pointed the finger at both sides of the political spectrum, the occult, drunken pranks, bankrupt celebrities, sham trials, and an incredibly effective hush campaign from the top rung of the political ladder. The jewels have never been recovered.
Via: The Greatest Unsolved Heist in Irish History – Atlas Obscura