In 1879, on the south side of the parish church in Knock, Co Mayo, fifteen people witnessed a vision of the Virgin Mary, St Joseph and St John the Evangelist appear through the pouring rain.

Knock Shrine is now one of the biggest Catholic pilgrimage sites in the world, with more than one and half million visiting each year.

The apparition reportedly lasted for two hours. It showed John leading Mary and Joseph in prayer. He was stood at an altar with a book open in his hands and a mitre on his head. There was a cross and a lamb on the altar with a group of angels were hovering above.

Mary was wearing a golden crown and had her eyes raised towards the sky with Joseph standing by her side. All three were dressed in white robes.

The incredible vision appeared at about 8pm on the 21st August 1879. It was witnessed by 15 people with ages ranging from 6 to 75 years old. All described the scene in the same way.

Two separate Commissions of Enquiry have since accepted the accounts to be trustworthy. A journalist named P.C.D. Warren interviewed several of the witnesses in the year after the vision.

He reported: “The persons who tell of those appearances and who solemnly declare the truth of what they assert, are numerous, respectable, and respected by their neighbours. Their answers are frank and civil, equally free from boldness and evasion, and their united testimonies constitute a mass of evidence which few impartial men will seek to discredit.”

A second commission in 1936 also found no reason to doubt the validity of the witness statements. Mary O’Connell was one of the two remaining witnesses alive at the time of the second inquiry. When questioned she said: “I am clear about everything I have said and I make this statement knowing I am going before my God.”

The incident is recognised by the Catholic Church as a modern-day miracle. Pope John Paul II visited the site in 1979 to celebrate the 100 year anniversary of the apparition.

The site has now been transformed into a large pilgrimage site for Catholics from around the world to visit. Much of the credit for this goes to a Catholic priest named James Horan. He was appointed as the parish priest in Knock in 1963. He realised the lack of amenities made it impossible for the site to cater for the thousands of pilgrims it attracted.

He ordered the building of a Basilica and was key in the planning for the construction of Knock airport.

The site now has five churches, a hotel and caravan and camping park, a museum, a café and a gift shop.

Pilgrims are invited to take part in prayer groups and vigils and are welcome to attend mass in the impressive 10,000 capacity cathedral. A guided tour of the site is also available.