The Celtic Cross
The Celtic Cross – a cross with arms of equal length encircled by a ring – is another Celtic symbol that possesses an array of possible meanings and interpretations. Some sources claim that this form of the cross predates Christianity, and is not an outgrowth of it. In Ireland, Wales, and Scotland, the earliest representations of this symbol, in the form of stone carvings, date from the seventh to ninth centuries.
Some possible interpretations of the quartered circle symbol is that it could represent a compass, and hence the four corners of the globe; additionally, it might stand for earth, air, fire, and water, the four elements of ancient times.
The unification of circle and cross may have been a way for the followers of early Christianity to make Christian symbols more palatable to the pagan peoples they were seeking to convert to the new faith. Some pagans held that the circle signified the moon, while a circle with a cross represented the sun. Adopting the Celtic cross as a religious symbol enabled early Christians to appeal to pagan worshippers of the sun and moon.
Some scholars feel that the Celtic cross may have its origins in the Chi Rho symbol introduced by the Roman emperor Constantine. "Chi" and rho" are the first two Greek letters in the word "Christ," and the letters overlap in a way similar to that of the intersecting circle and cross that form the Celtic cross.
The earliest Celtic crosses did not stand erect; they were instead carved
into large rocks that lay flat on 
the ground. Later, High Crosses, a version
that stands upright, became dominant. Frequently used as ornamentation on
Celtic crosses are the Celtic Knot, the figures
of animals or birds, spirals, leaves and branches, and Biblical references.
Today, High Crosses can be seen far and wide across the landscape of Ireland. To see them, you will need to visit the centuries-old ruins of some of the country’s most revered monasteries, which date back to the sixth century. Some of the best-preserved examples of High Crosses still stand in County Meath, at the Kells monastery; in County Offaly, at the ruins of Clonmacnois; and in County Louth, at Monasterboice. On these, as well as other Celtic Cross carvings, you are likely to see images from pagan mythology alongside representations of people and events taken from the pages of the Old and New Testaments.