A well written article I've just come across on Newgrange and O'Kelly's linking up the roof box with solstice. There are quotes from Seamus Heaney's "Dream of Solstice" poem,
http://sluggerotoole.com/2010/12/19/a-seasonal-gift-to-the-benevolent-spirits/
The whole poem can be found here......
http://www.themodernantiquarian.com/forum/?thread=23046&message=259467