A vast mosaic containing more than 1.5 million pieces has been unveiled at the shrine of Knock in Ireland. The mosaic, illustrated by the Irish artist PJ Lynch, depicts the apparition of the Virgin Mary to 15 witnesses in 1879. At 46ft high, and the same width, it is one of the largest mosaics of its kind in Europe. In his homily at Mass on Sunday, when the mosaic was unveiled, the Archbishop of Tuam Michael Neary said: "We thank God for this marvellous day as we unveil this magnificent mosaic, which conveys in such breath-taking beauty the mystery of the Apparition." The apparition took place on the evening of 21 August 1879, when 15 people at the parish church in Knock saw the Blessed Virgin Mary, St Joseph and St John the Evangelist standing beside an altar surrounded by angels. A cross and a lamb stood on the altar. After a Commission of Enquiry declared that the apparition was trustworthy, and international media reported the story, Knock became a major pilgrimage site. Today about one and a quarter million people are estimated to visit every year. St John Paul II visited the site and celebrated Mass on the centenary anniversary in 1979. The installation of the mosaic comes at the end of an 18-month refurbishment project. It has been accompanied by a renewal process, ‘Witness to Hope’, launched by the parish priest Fr Richard Gibbons.