It measures between 79 and 85 metres in diameter with a height of 11 to 13m. The roof box was discovered in 1963 during excavations. It is 2.4m back from the passage mouth and rests on the front part of the passage roof. This phenonomen was discovered in 1967. This sunlight illuminated the chamber.
Sunlight also appears in the chamber for a week before and after the winter solstice.
An ancient Irish tomb may have been built with a light chamber aligned not only to the Sun, but to the Moon as well. Building it would have required many years of observations of the motions of the Moon by the tomb's architects. The tomb could also explain the Moon-inspired names of local landmarks. There are 97 kerbstones around the base of the mound. They range in length between 1.7m and 4.7m.