On August 21, 1879, Margaret Beirne, a resident of Cnoc Mhuire, was sent by her brother to lock up the church for the evening. When she was ready to leave, she noticed a strange brightness hovering over the church. Margaret had other things on her mind, and didn't tell anyone what she saw. Around the same time, another member of the Beirne family, Mary, was leaving from a visit to the church's housekeeper, and stopped with the housekeeper at the gables, where they could see the church. Mary replied:


"Oh look at the statues! Why didn't you tell me the priest got new statues for the chapel?"
The housekeeper responded that she knew nothing of the priest getting new statues. So, they both went for a closer look, and Mary Beirne said:


"But they are not statues, they're moving. It's the Blessed Virgin!"
Thirteen others also came and saw the beautiful woman, clothed in white garments, wearing a brilliant crown. Her hands were raised as if in prayer. All knew that it was Mary, the Mother of Jesus, Queen of Angels. On the right of Our Lady stood St. Joseph, his head inclined toward her. On her left stood St. John the Evangelist, dressed as a bishop. To the left of St. John stood an altar which had a lamb and a cross surrounded by angels on it. The vision lasted about two hours. People who were not at the apparition site reported that they saw a bright light illuminating the area where the church was. Many of the sick were healed upon visiting the church at Knock.

http://www.catholicculture.org/culture/liturgicalyear/calendar/day.cfm?date=2015-08-21