I tend to agree with Droood on this. Danu / Anu seems to be a primordial river goddess, which in Celtic myths seems to be related to the spirit of the land. Look at the story of Boann (the river Boyne), as precised here :
http://en.wikipedia.org/wiki/Boann, to see the link. She is married to Nuada the King of the Irish Gods. This link of a River Goddess to a male Tribal God seems to be typical of the Celtic worldview. In this respect each land of the Celts would have had their own primordial river goddess linked to their own tribal King of the Gods.
Gaia is the Greek mother earth - so encompassing the whole of the earth, the rivers etc... and is so integral to the Classical Greco-Roman wordview.
In my experience this is the risk with trying to equate a goddess from one worldview with another. Certainly within the Romano-British world it seems that they had the same problems, for example Nodens (or Nuada in Irish) is equated with the Roman Mars, Neptune and Silvanus. In trying to abstract the two goddesses into a single archetype you miss the nuances and subtleties that are the detail, thus the wisdom, in each of the mythologies. I have been through a similar process with the Germanic and Celtic and ultimately came to realise that you lose more in the details than you gain in the commonality.
Most dear is fire to the sons of men,
most sweet the sight of the sun;
good is health if one can but keep it,
and to live a life without shame. (Havamal 68)
http://gewessiman.blogspot.co.uk