One things for sure, Celtic religion certainly had staying power. Now that we tend to rule out mass invasions, it's difficult to rule out many Celtic gods having roots in the Bronze Age (at a local level), however, there are certainly signs of Bronze Age burial and other possibly ritual sites being ignored and in some cases slighted in the Iron Age, which could indicate a change in religious practice.

I think that the foundation of the Celtic gods may have co-incided with the discovery of metal (not sure thought). I thought of this during the Bronze Age conf.

Prior to the discovery of metal, there is no doubt that man worshipped gods (cos it's what we do). However these gods will have represented the accepted interpretation of the world around them. Interpreting those "elements" that were important to life as being things to worship if you like.

Them along comes these upstart tribes with metal objects, these don't fit into the pre metal age Pantheon, and to a very great extent challenged it at its highest level. I'm sure there will have been lots of local meetings where these tribes were branded as heretics and would have been cast out. Not able to to follow the established church, and filled with the loss of inocence that follows when a new discovery replaces a previous knowledge, perhaps these few tribes developed there own interpretation of god(s), fitting the new powerful metal based gods into the existing pantheon but in there isolation this pantheon developed on a different track, along the line losing links to the "stone ways" such as building large static monuments of stone. Perhaps this was forced on them by the fact they were outcasts and had a nomadic lifestyle. It may be that these are the people we see emerging as the beaker people - the evangelists of the metal cult.

It's always nice to speculate!