Just a quick addendum. Danu is not attested within Celtic writing. The idea comes from a proposed root for Tuatha Dé Danann, but the extrapolation of mother-Goddess Danu seems to be a much later (Victorian) invention which bears little resemblance to the text itself. Which leaves Danu as a Scythian/Vedic (Indo-Iranian) word. Given the geographical position of the Scythians and the rivers bearing the Danu/Don prefix a Scythian root is far more likely than a Celtic one.