The spiral design comes from the cross-slab at Aberlemno, Angus, in Scotland. A good working understanding of the use of the triskele (an archetypal symbol of power, later called the 'legs of man') is needed to conceive such a design.
When early man observed the beauty of nature's spirals it is not surprising that the spiral would become a potent symbol for creation and growth. It is the only decorative motive used in Christian Celtic art proven to have its roots in the preceding pagan period, the best examples of which are found on stone monuments such as the decorated kerb-stone at the entrance to the burial chamber at Newgrange, Co. Meath, in Ireland, which dates from around 3000 BC.
(from the book, Knotwork and Spirals by Courtney Davis)