Today is the winter solstice – when the sun is over Capricorn – and if visitors are lucky will shine directly into the Stone Age chamber in the Boyne Valley.
The passage tomb was constructed 5,000 years ago and is a UNESCO World Heritage site.
Thousands of people are expected to descend on the Meath location for the shortest day of the year – but only the lucky lottery winners will be inside the historic chamber.
Met Éireann says rain is predicted so the sun isn't likely to shine through.
