Coven of the Unicorn's Realm
The Celtic accepted the following division being; three elements divided into three realms.
Air: Wind Water: Sea Earth: Rock Sky River Soil Storm Spring Wood
There are many ways that these can be divided, and variations on the system.
Realm "element" physical spiritual Land: earth flesh body stone bone strength salt nerves fate Sea: water blood force rain fluids desire cloud brain consciousness Sky: sun face talent stars eyes path wind breath soul
These can be further broken down into three divisions of head/heart/hand(body), the three harp strains, and the three cauldrons, but those divisions aren't necessary. As I said, all of this is flexible and different sources from different periods will list different "elements" in different orders.
"Element" is misleading anyway. Rather than being a primal creative component in the universe (as earth/air/fire/water in ceremonial magick), "element" instead implies "created things within the universe." These are, as well, philosophical rather than physical divisions within Celtic religion.