How the Celtic Cross Form Came to Be
The Celtic cross, as represented by the few remaining standing crosses in Ireland, flourished as a religious and art form during the early Middle Ages. During this time, Ireland slowly converted from druidism and other so-called “pagan” religions to Christianity, though the conversion was never complete. In fighting for the souls of the Irish, Christian authorities used a tried-and-true tactic: rather than forcing the locals to discard their religion, they assimilated it.
The Celtic cross is a cross superimposed on a circle (not, significantly, the other way around). The circle is thought to represent the sun, the object of many ancient religions; a representation such as is seen on the Celtic cross is called a nimbus. Though no writings survive that explicitly say it, it is thought that the cross-with-nimbus symbolized the acceptance of the old druidic religion (by showing the nimbus) but also its being eclipsed by the new Christian religion (since the nimbus is clearly behind the cross).
Of course, there was a simple practical advantage to this design as well: the circle served to buttress the cross’s horizontal arms. In fact, Celtic crosses without the nimbus usually did not survive long; weathering and age caused such crosses to lose their arms or heads.
The Art of the Celtic Cross
The intricate filigree-like carvings on the surface of Celtic crosses are of a style known as “insular art,” which literally means “art of the islands.” This style combined motifs from earlier runic art with intricate swirls and patterns to form a visually striking mosaic. This made the standing crosses very visually impressive, particularly in a hardscrabble medieval world where very few things were ornate, decorative, or beautiful for their own sake.
You can also see insular art in manuscripts such as the Book of Kells and the Lindisfarne Gospels. If you study it closely, you might be struck by how abstract the art is; even when people are clearly depicted, they are not drawn in a factual style. The more important element is that such depictions be visually arresting.
The Viking Influence
The flourishing of insular art is thought to have been stifled by the Viking raids and invasions of the 8th, 9th, and 10th centuries. At first, the invaders looted, killed, and took slaves, but later, they came back to stay; Dublin and Waterford, Ireland’s first two cities, were founded by the Vikings. They brought their own decorative styles and versions of Christianity with them, and the old Celtic/Druidic form of Christianity was supplanted.
Ireland during the early medieval period was not a unified country by any means; it was a collection of small, often mutually hostile kingdoms and chiefdoms. Thus, when the Viking raids came, first to the west coast and then to the rest of the island, there was no way to organize a unified force to resist them. The Vikings took home as loot many examples of insular art, such as manuscripts, metalwork, and wall hangings; very few such artifacts have survived, as the Vikings also usually destroyed what they could not carry back with them.
The development, both cultural and economic, of Ireland was retarded for centuries by the Vikings; many coastal settlements were abandoned after Viking raids. It was simply too much to painfully build up a small measure of wealth and prosperity and then see the fierce Viking warriors swoop in from the sea and take or destroy it all. Many Irish moved inland, out of the reach of the raiders.