Knock has become a popular destination for pilgrims
An Irish eye surgeon has said an "unprecedented" rise in the number of cases of an eye condition could be directly related to people staring at the sun at a holy shrine.
Thousands of people have been travelling to the County Mayo pilgrimage shrine of Knock after hearing that the Virgin Mary would appear there.
Some claimed to have seen the sun "dancing in the sky".
Dr Eamonn O'Donoghue, a consultant ophthalmologist surgeon told the Irish Times that the hospital would usually see one case of solar retinopathy "at most" per year.
However, there have been five such cases so far this year, all of them linked to events at Knock.
Dr O'Donoghue, who is based at University Hospital, Galway, said people needed to be warned of the condition as it was "potentially very, very dangerous" and could cause long-term damage to the most vulnerable part of the eye.
"These people came in because they have had a significant reduction in their vision and they could very well be a smaller representative sample," Dr O'Donoghue said.
He added that two of those who had presented to the hospital had also reported that other members of their families had suffered visual damage.
Staring at the sun can cause retinal damage
"It can potentially lead to blindness with prolonged exposure," Dr O'Donoghue said.
While he was unaware how the events were organised, he said, it would be "profoundly irresponsible" for anyone to encourage people to stare at the sun.
A crowd of about 7,000 people gathered at the shrine of the Virgin Mary on Saturday 31 October.
At this and at another event on 11 October, some people claimed to have seen the sun "dancing in the sky".
Solar retinopathy, or eclipse retinopathy as it is also known, can cause a significant reduction in vision. It can also lead to altered images, altered colour perception and blind spots.
While most people will recover their vision within six months, solar retinopathy has the potential to have a long-term degenerative effect on the retina.
He said that reports of people seeing colours dancing in front of the sun could also be explained by the condition, describing it as "sort of a cheap trick".
"If you stare at the sun for long enough you're going to get some visual disturbances. Not only will you get reduced vision but also a condition called metamorphopsia," he said, adding that this could explain such visual alterations.
"Since the time of Galileo people have known that looking directly at the sun can do damage to your eyes," Dr O'Donoghue said.