Caption: The entrance to Newgrange as it appeared long before reconstruction in the 1970s. Photo from Celtic Myth and Legend (1905) by Charles Squire (fl. 1900). See also Early Celtic Scholarship and the Romantic Celt: Charles Squire by Juliette Wood, Cardiff University.
Features:
The triple spiral was used in Neolithic Ireland, Bronze-Age Ireland, and Iron-Age Ireland when Celts arrived probably before or circa the 6th century BCE as aforesaid.
At sunrise on the winter solstice, sunlight beams down the main passageway and illuminates a chamber floor (see Wikipedia: Newgrange: Purpose). This alignment was almost certainly planned.