⁶⁸O'Kelly, Michael J., and Claire O'Kelly. Newgrange: Archaeology, Art, and Legend. London: Thames and Hudson, 1982. 117.
The author's estimate of the number of years required to build the tomb presumes that they were able to work for two months each year, after the spring planting season. His figures are based upon the earlier work of Frank Mitchell. (Mitchell, Frank. The Irish Landscape. London: Collins, 1976. 130.)