Pilgrims at Knock Shrine in Co Mayo have been warned that staring at the sun in the hope of seeing apparitions could seriously damage their eyesight.
Thousands of people flocked to pray at the shrine in October, where they claimed to have seen the sun shimmering and dancing in the sky.
But University College Hospital Galway said it has seen five cases of the damaging eye condition solar retinopathy this year, with four directly linked to people gazing at the sun.
Dr Eamonn O'Donoghue, consultant ophthalmologist surgeon at UCHG, said the hospital would normally see just one case a year on average.
'All of these people were under the impression they would see a strange phenomenon if they looked at the sun and it is associated with events at Knock,' Dr O'Donoghue said.
'It is a shame to see Knock being misused this way.'
Knock, where Catholics believe the Virgin Mary, St Joseph and St John the Evangelist appeared in August 1879, has been a place of pilgrimage for decades.
People suffering from solar retinopathy have had the tissue at centre of their retina, known as the macular, effectively burned.
'If you get a burn to the macular you are going to see bizarre phenomena,' the doctor said.
'This is a tissue of very limited capacity for self-repair.'
Normal vision could return after a few weeks for mild damage, but severe solar retinopathy will permanently damage a person's vision.
Thousands of people turned up at Knock on two occasions in October hoping to see a visitation from the Virgin Mary.
It had been predicted by Dublin man Joe Coleman, who is a self-proclaimed spiritual healer.
Mr Coleman has predicted another vision in Knock this Saturday.
Archbishop of Tuam Michael Neary has criticised the predictions made by Mr Coleman.
Archbishop Neary said: 'They risk misleading God's people and undermining faith. For this reason such events are to be regretted rather than encouraged.'