We tend to associate the Celts with Ireland, since that's the locale in which their music and traditions flow most completely today. But in fact the Celtic tribes covered most of Europe in the immediately pre-Christian centuries. Like other tribal societies, theirs was a culture of war, headed by a king and emphasizing the bonds and loyalties of kinship.
It appears that women played significant roles in Celtic society and leadership, and that their pre-Christian religious beliefs and rituals emphasized the sacredness of the land and the natural world. Those aspects of their culture were incorporated into Christianity as they adopted it and are reflected in their artwork, increasingly popular today. Although the Celts were eventually subsumed under Roman and Germanic control, their influence extends far beyond the stone monuments still scattered across Great Britain, Ireland, and northern France.
When my daughter and I visited the Canadian Maritimes last summer, we saw evidence of Celtic influence scattered across the countryside. And whenever a group of children departs from the North Carolina camp where she is working, the counselors sing them out with the Celt blessing:
May the road rise to meet you.
A terrific website on Celtic Christianity can be found here.
Images: A map of Celtic Europe, a pattern from a cemetery monument here, an illumination from the Book of Kells.