I read Quinn's book a couple of years ago and saw the film he made about 20 years back. Some intriguing hypotheses - part of his motivation was to try and debunk the idea of a Celtic 'Race'. He supposes that cultural influences from North Africa made their way to North-Western Europe by sea and that early Gnostic Christianity arrived the same way. Not implausible. He also goes into linguistic similarities but I don't know enough to say if he's right or not. I must have another look at it.