Celtic Christianity falls into two clear categories: historical and modern revival attempts.The Celts are the peoples of Ireland, Scotland, Wales, parts of England and Brittany. The Celtic Church was unique to these peoples, and is reputed to be one of the oldest in the Christian world. Tradition relates that it was founded by Saint Joseph of Arimathea in 37 AD, in Britain, in a place which is now called Glastonbury.
Another disciple, Saint Aristobulos, arrived in the British Isles in 63 AD. By all accounts, the Celtic Church remained free of all temporal power and poor. Monasticism played a capital role and gave us Saints Patrick, Brigit, Columba, Brendan, Samson, Amand, Fare, Columban and many others. Many important fragments remain of the Celtic liturgy, and the Use of Sarum, despite being a Norman variant of the Roman Rite, was to some extent influenced by the Celtic tradition.
This article seems to give an interesting account of the history, doctrines and traditions of Celtic Christianity. I have done no independent study of Celtic Christianity. We learn that the Gregorian Reforms (1150–80) were largely responsible for absorbing the Celtic Christians into the Roman mainstream.
It seems an attractive aspect of “Northern Catholicism”.
In modern times, a number of communities have sprung up inspired by a more or less authentic understanding of what Celtic Christianity was like according to available sources. The one that springs to my mind is the “non-canonical”Celtic Orthodox Church.
For others, I suggest going to Google and typing in “Celtic Christianity”, “Celtic Catholic Church”, “Celtic Orthodox Church” and Celtic-just-about-anything-else. You will certainly discover a number of New Age and “inclusive” communities, which I am not recommending in any way. There are even some real kooks out there, so discern and use your brains!