Ireland’s days of terror may not quite be over. Police found a “viable improvised explosive device” on a bus near Dublin Monday night—the eve of Queen Elizabeth II’s historic visit to the country. The bomb, found in the luggage department of a bus in the outskirts of Maynooth, was detonated in a controlled explosion. The queen’s trip is the first by a British monarch since Ireland became independent. It takes place amid an unprecedented 4,000-person, €30 million security operation, which includes a “ring of steel” around Dublin. On Monday, a dissident Irish republican group issued a bomb threat for central London.