²⁷Harbison, Peter. The Archaeology of Ireland. New York: Scribner, 1976.
According to Carleton Jones, "As the great antiquity of humans became recognised in the nineteenth century, another related idea was also gradually accepted, the idea of successive ages of stone, bronze and then iron. In Ireland, this 'three- age system' was introduced by J.J.A. Worsaae, a Danish archaeologist who was invited to lecture to the Royal Irish Academy. One consequence of Worsaae's visit in 1846 was that an analogy was drawn between the Danish megalithic tombs and Irish megalithic tombs with the result that Irish megalithic tombs were now correctly attributed to the 'Stone Age' rather than to a generalised pagan past as they had been previously." (Jones, Carleton. Temples of Stone: Exploring the Megalithic Tombs of Ireland. Cork: Collins, 2007. 3-7.)