Byrne, Mary (1850–1936), witness of the Knock apparition, was the eldest daughter among three children of Dominick Byrne of Co. Mayo and his wife Margaret Bourke, and grew to be a tall, thin woman, with black hair and a pleasant voice. The defining moment in her life occurred when she was 29, although in depositions she deducted three years from her age; she corrected this in a statement in 1932. On 21 August 1879 she was brought from her mother's house at 8.15 p.m. by Mary McCloughlin, housekeeper of the parish priest, Archdeacon Bartholomew Cavanagh (qv), to witness an unusual sight at the gable of the church. Investigating, Mary witnessed three figures hovering about two feet above the ground. McCloughlin was certain that one of the figures was clearly the Virgin Mary, another Joseph. It was Mary who identified the third figure as St John because of his resemblance to a statue in a church at Lecanvey, near Westport, Co. Mayo. Mary then went to a number of nearby houses and assembled a group of fifteen people to witness the apparition. These included her younger brother Dominick, her sister Margaret, and her widowed mother. Archdeacon Cavanagh refused to leave his house despite the efforts of his housekeeper, whom he considered unreliable. The story of Knock soon spread, and in the following years Mary came to be seen as the most important of the official witnesses. On the first anniversary of the apparition she carried a banner of the Virgin Mary in the commemorative procession. Her lifelong regret was that Cavanagh had not witnessed the apparition.
On 1 July 1882 she married James O'Connell from the nearby village of Becan, who was the same age. They had five sons and a daughter. Two of the sons, Patrick and Dominick, became policemen: the former in Chicago, the latter with the RIC in Belfast. When her husband died (1926), Mary continued to live near Knock with her eldest son, James. She was a pious and patient woman, never complaining when busloads of pilgrims would visit her house to touch her. She insisted that if she closed her eyes and opened them again she could see the vision. A great talker, she would curtly tell people to say their prayers. She gave a deposition on the apparition in 1932 and never wavered from her earlier testimony. On 27 January 1936, in her eighty-sixth year and knowing she had not long to live, she signed a sworn statement affirming what she saw at Knock. In August 1936 she was examined by the second commission of Knock and impressed them with the clarity of her recollections. She died 19 October 1936 and was buried in Knock cemetery beside her husband.