Catholic pilgrims have suffered eye damage after staring at the sun in the hope of witnessing an apparition of the Virgin Mary, a doctor said.

On one occasion in October, some 10,000 people gathered at the Knock shrine in northwestern Ireland hoping to see Mary, despite pleas from an archbishop to ignore invitations to the event by a self-proclaimed spiritual healer.

Some of those present said they had seen Mary, venerated by Christians as the mother of Jesus, and attributed her presence to the sun suddenly breaking through the clouds, changing colour, appearing to come closer or spinning in the sky.

Eamonn O'Donoghue, an ophthalmologist at University College Hospital Galway in the west of Ireland, said he had several patients whose retina had been burnt by the sun during a visit to Knock.

"All of these people were under the impression that they would see strange phenomena if they looked at the sun, it is associated with events at Knock," O'Donoghue told public radio RTE.

"If you get a burn to your macula (a part of the retina) you are going to see bizarre visual phenomena," he said, comparing the effects to frequently observed damage from watching solar eclipses without adequate protection.

The patients could expect an at least partial recovery within months, he said.