Eoghan Harris in the Sunday Independent newspaper says his grandfather, a farmer from near the area, believed like many at the time that it was two local policemen with a magic lantern, a device that was widespread in the 19th century which allowed a small lightbox to project an image on a wall, who did it.
According to the site Catholictradition.org, the vision began on the evening of August 21, 1879. Mary McLoughlin, the housekeeper to the parish priest of Knock, County Mayo, Ireland, was astonished to see the outside south wall of the church bathed in a mysterious light; there were three figures standing in front of the wall, which she mistook for replacements of the stone figures destroyed in a storm.
She rushed through the rain to her friend Margaret Byrne's house. After a half hour Mary decided to leave and Margaret's sister Mary agreed to walk home with her.
As they passed the church they saw and amazing vision very clearly: Standing out from the gable and to the west of it appeared the Blessed Virgin, St. Joseph and St. John. The figure of the Blessed Virgin was life-size, while the others seemed to be neither as large nor as tall.
They stood a little away from the gable wall about two feet from the ground. The Virgin was erect with her eyes toward Heaven, and she was wearing a large white cloak hanging in full folds. On her head was a large crown.
Mary Byrne ran to tell her family while Mary McLoughlin gazed at the apparition. Soon a crowd gathered and all saw the apparition. The parish priest, Archdeacon Cavanaugh, did not come out, however, and his absence was a disappointment to the devout villagers.
Among the witnesses were Patrick Hill and John Curry. As Patrick later described the scene: “The figures were fully rounded, as if they had a body and life. They did not speak but, as we drew near, they retreated a little towards the wall.” Patrick reported that he got close enough to make out the words in the book held by the figure of St. John.
An old woman named Bridget Trench drew closer to embrace the feet of the Virgin, but the figure seemed always beyond reach. Others out in the fields and some distance away saw a strange light around the church. The vision lasted for about three hours and then faded.
The next day a group of villagers went to see the priest, who accepted the their report as genuine. He wrote to the diocesan Bishop of Tuam, then the Church set up a commission to interview a number of the people claiming to witness the apparition.
The diocesan hierarchy was not convinced, and some members of the commission ridiculed the visionaries, alleging they were victims of a hoax perpetrated by the local Protestant constable.
But the ordinary people were not so skeptical, and the first pilgrimages to Knock began in 1880.