What is the story surrounding Our Lady of Knock?

On a rainy August evening in 1879, in the town of Knock in County Mayo, Ireland, several witnesses saw three figures standing outside the parish church. They described the three figures as Mary, standing in the center; St. Joseph, on her right; and St. John the Evangelist, standing on Mary’s left. Behind the three was an altar upon which rested a lamb. The scene lasted for nearly two hours, and none of the three figures spoke or moved. In all, fifteen witnesses reported seeing the apparition.
Traditionally, the apparitions at Knock have attracted even more skepticism than usually greets Marian apparitions because they are so “outside of the mold” of most visions: Mary appeared not singly, but with others, and she did not speak to those who saw her. That said, Pope John Paul II appeared to endorse the apparition when he made a pilgrimage to Knock for its 100th anniversary in 1979, and the shrine is a popular pilgrimage site, as well as the source of many reported cures and healings. As with every apparition story, Knock is very meaningful to some Catholics and not to others. Every Catholic is ultimately free to decide what role, if any, these apparitions play in his /her faith life.