I've been re-reading one of the old stories about Newgrange. It's called Altram Tíge Dá Medar, which means something like "the fosterage of the houses of the two drinking vessels". It There are some very interesting aspects of the story which may describe aspects of the monument.
For a summary of ATDM, see this page on my website.
In the story, two girls are born, one to Manannán (a Tuatha Dé Danann chief deity) and his wife, and one to Dichu (the steward of Sidh an Bhrogha/Newgrange). Manannán's daughter is named Curcog (coirceog in modern Irish), which means "beehive". In fact, it means conical or cone-shaped too.
This is interesting, not least because that is the shape of the ceiling of the chamber of Newgrange!
Of further intrigue is the fact that Oengus made a "beautiful shapely grianán" for the two maidens, Curcog and Eithne, whom he fostered. A grianán is a sun chamber or sunny bower, a gynaeceum or soller. It is, one might say, the perfect description of the chamber of Newgrange at the time of sunrise on winter solstice!
In the above picture, taken from an animation made about Newgrange by Fáilte Ireland (for its Ireland's Ancient East advertising), we can see the cone- or beehive-shaped chamber (curcog/coirceog) of Newgrange being lit up by the rising winter sun.
When Manannán visits Newgrange, the "excellent (bright) Sidh an Bhrogha was freshly strewn with rushes before Manannán". Does this, perhaps, give us an indication of how the monument might have been decorated for special occasions?
There is no doubt that, while this story has been considerably intruded upon by Christian scribes, some elements of it are very ancient. We are left in no doubt that Sidh an Bhrogha / Newgrange was a place where the pre-Christian deities of old were gathered:
"But the powerful ones of the Tuatha Dé Danann, and the nobles of the Land of Promise were all there, and there was not a prince or a lord of them all but felt longing and envy for that house."