Jaze well this could be my first kind of dodgy ruleswise post. I dunno how its goin to be recieved.
Also, the actual knowledge of what there beliefs is not actually well known, as not much of it was written down, as it was passed though a oral tradition, and this has been lost. However, even if we where to develop a time machine and learn what that knowledge is, it may very well be unique to that clan or group.
Id disagree there, my view is there is no need to reconstruct polytheism because its always been a part of the traditional cultures. Certainly in Ireland anyway and Id assume cos we are geographically close and not VASTLY different in our history that Wales and Scotland would be the same. The way of viewing deities is cultural and culture stretches back to time imemmorial. Whether youre christian or aknowledge that its polytheism it doesnt matter cos its the same practice.
A Pagan religion from the Iron age though, I agree that needs reconstructing to build foundations. It doesnt exist today but it will take some imagination because the religion was never codified and a reconstructionist has to do unprecidented work to codify it. Ideally they would be guided by the framework of traditional culture of one form or another be it their own countries or the country of origins.
So what does that leave us?
you have a large assortment of Celtic deities, and find which ones you have a connection to, discover the local deities, that is the easy part.
Now you faced with the daunting tasks of recreating the rituals needed, and that is a tough road, but there is personal rites, and community rites, and once established, once you have discovered and maintained a holy place, celbrate, then teach your children, and in a generation or two you might have your own clan of Celtic faith existing, of course, if you find fellow followers, it might be sooner!
All those things already exist in the same place they did in the Iron Age. The Traditional Cultures. With the possible exception of rituals for energy work cos thats atleast only 18th century and completely British. If you want those things embrace your local culture and add its shared knowledge and values to the religion, it would certainly help the religion function in a realistic way. Or move to Ireland or somewhere and live in a traditional community. So long as you dont walk around telling people that what they do is pagan cos its related to pre christian idea's and you'll just be someone really really interested in Irish Culture to them.
James