The triple spiral or triskele is a Celtic and pre-Celtic symbol found on a number of Irish Megalithic and Neolithic sites, most notably inside the Newgrange passage tomb, on the entrance stone, and on some of the curbstones surrounding the mound.
Believed by many to be an ancient symbol of pre-Celtic and Celtic beliefs, the triple spiral appears in various forms in pre-Celtic and Celtic art, with the earliest examples having been carved on pre-Celtic stone monuments, and later examples found in the Celtic Christian illuminated manuscripts of Insular art. The triple spiral was possibly the precursor to the later triskele design found in the manuscripts.
What the symbol meant to the pagans who built Newgrange and other monuments is unknown. As Christianity came into the forefront in Ireland before the 5th century, AD, the triskele took on new meaning, as a symbol of the Trinity: Father, Son, and Holy Spirit, and therefore, also a symbol of eternity. Its popularity continues today as a decorative symbol of faith for Christians of Celtic descent around the world. Neopagan religions such as Celtic Reconstructionist Paganism and Wicca use the symbol to represent a variety of triplicities from their belief systems.
The triple spiral is one of the main symbols of Celtic Reconstructionist Paganism, often standing for the "three realms" - Land, Sea and Sky, or for one of a number of deities who are described in the lore as "threefold" or triadic. The god Manannán is probably most often the one symbolized by the triskele, though some also use it for the goddess Brighid. Some Celtic-inspired Wiccans also use the triple spiral symbol, most often to represent the concept of the triple goddess.
According to Uriel's Machine by Knight and Lomas (2003) the triple spiral may represent the nine-month period of human pregnancy, since the sun takes a fourth of a year to go from the celestial equator (an equinox) to extreme north or south declination (a solstice), and vice versa. During each three-month period, the sun's path across the sky appears to form a closely wound quasi-helical shape, which can be likened to a spiral, so that three spirals could represent nine months, providing an explanation for a link between fertility and the triple-spiral symbol.
One decorative version of a "wheeled" form of the triple spiral symbol, sometimes considered a solar symbol
See also 
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- Bonewits, Isaac (2006) Bonewits's Essential Guide to Druidism. New York, Kensington Publishing Group ISBN 0-8065-2710-2. p.132: [Among Celtic Reconstructionists] "...An Thríbhís Mhòr (the great triple spiral) came into common use to refer to the three realms." Also p. 134: [On CRs ] "Using Celtic symbols such as triskeles and spirals"