The warning comes after four people have been treated for the condition called solar retinopathy
following the last gathering at Knock on October 31.
The sun's rays can cause solar retinopathy
, in which the macula - the most sensitive part of the retina - is permanently damaged and cannot be treated.
But some were told that they had suffered solar retinopathy
, where looking directly at the sun's ultra-violet and infra-red rays for too long has burned the macula - the most sensitive part of the retina, where light rays converge and objects come into sharp focus.
Jonathan Dowler, consultant ophthalmic surgeon at Moorfields Eye Hospital in London, said: "We are quite pleased the number of people suffering solar retinopathy
(eye damage) is relatively low.