Celtic Crosses dot hundreds of cemeteries across Ireland and Scotland, as well as Wales, England, Europe, and beyond. Few symbols are as recognizable as the Celtic Cross as the embodiment of Celtic Christianity. It is popularly believed that St. Patrick introduced the Celtic Cross in Ireland, during his conversion of the kings from paganism to Christianity. Some also believe it was St. Columba or St. Declan who introduced it. Other theories site construction strength to the design – the circle strengthened the cross beams, preventing breakage or destruction by the elements or time.
While the Celtic Cross is certainly a Christian symbol, it has its roots in ancient pagan beliefs at the same time.
The stone circle at Calanais, on the Isle of Lewis in Scotland, is formed in a rough circle, with an even-armed cross within it. This is believed to be a sun symbol to the creators of the stone circle, which became a sacred shape to the Celts.
St Patrick is said to have taken this ancient sun symbol and extended one of the lengths to form a melding of the Christian Cross and the sun symbol, and thus the birth of the Celtic Cross.
The even-armed cross within a circle has been ascribed many meanings by many groups and cultures. One such meaning is that of the stages of the day: morning, noon, evening, midnight. Another possibility includes the meeting places of the divine energy, of self, nature, wisdom and divinity. Of course, obvious relations such as east, north, south and west; or earth, air, water and fire can also be derived from the shape. Even the Native Americans used this as a symbol for their Medicine Wheel. The sun wheel has also been called Odin’s Cross, a symbol in Norse Mythology.
Source (Irish Fireside)