The Celtic Cross, which also usually incorporates some kind of knotwork, can be seen as a synthesis of ancient mystical beliefs with later Christian faith interpretation. Mythical creatures such as dragons are also very popular tattoo images.
The Celtic knot, in particular, represents infinity in a way similar to the Chinese Yin-Yang symbol and the caduceus of Western esoteric tradition. Recently it has been pointed out that ancient Celtic spirituality was actually a form of shamanism. I think that Celtic art and tattoo stencil machine symbolism is popular with tattoo aficionados because it overlaps with an interest in things mystical and sometimes related to shamanism. This does not diminish it in any way; I am only pointing out an interesting fact about tattoo history. The Celtic tattoo phenomenon, on the other hand, is a question of bringing traditional Celtic art into a new medium.
