Until the end of the 19th century Knock was a quiet village on a windswept ridge of a blanket bog. One evening in August 1879 two local women and 13 others saw the figures of the Blessed Virgin Mary, St. Joseph and St. John bathed in light by the south gable of the church. The appearance has since been validated by the Catholic Church. Consequently the village attracts over one and a half million pilgrims each year. Among them have been Pope John Paul II in 1979, and Mother Teresa in 1993.
The original church was built in 1828 with stained glass windows by Harry Clarke. The gable end where the vision was observed is now a glass-enclosed oratory.