The Celts offer a unique source of philosophy to those that study them.
The Celts had a concrete philosophy that was passed down the generations through the oral tradition in the form of symbol, metaphor and verse. Academics seem to think that the philosophy of the Celts has been lost with the Druids, but they forget that the Bard was part of the same class of philosophers as the Druids, who retained the philosophy in their oral tradition.
Under the influence of Christianity some Druids and Bards became Christian priests, evident in that only these types could have transferred such Celtic knowledge to writing. There exists a vast body of knowledge that remains untranslated from the ancient Celtic tongues into English, since there is only a few translators capable of translating the material. What has been translated provides a glimpse of how the Celt viewed the world and their place in it.
The Voyage of Bran Mac Febal deals with the Celtic philosophy of the afterlife. The Celts are part of an ancient people who share the genetic Haplogroup R1B (Y-DNA) which pushes them back to the time of the hunter gatherers, before civilisation. The belief systems of the afterlife would have been oral over tens of thousands of years, and would be held in common with many other ancient peoples.
I know of at least three works that Bran is mentioned in, which suggests he is a bridge between the “otherworld” and the material world. In the Arthurian stories Bran becomes the Fisher King, guardian of the Holy Grail. Bran is mentioned in the works: Voyage of Bran Mac Febal, Bran and Branwen, and the Battle of the Trees.
The Celt believed in reincarnation, you died, and for a short time you travelled through the otherworld, to be reborn again in a new body. The Celts believed everything had a spirit, and the Druids acted in the roles of shamans to deal with these spirits. The veil between the material world and the otherworld was thin, and you get the impression of a leaky bucket where the otherworld and its denizens constantly overlap and leak into the material world.
There was two main methods of entry into the otherworld: death, or shamanic travel, normally with the help of an archetype like Bran.
The Voyage of Bran Mac Febal is a friendly version of the Celtic otherworld, the scribe even manages to include Jesus in the story. In other stories the Celtic otherworld and many of its denizens can be raw, wild and unfriendly; especially to gatecrashers with bad intent to steal from them. An example is how the birds sing Bran to sleep, a beautiful way to describe death; in the Basque country the stories of death may be of a creature dragging you screaming into a well.
It is best to think of the Celtic world view as three circles: the outer (material world), symbolised as Ireland in this story; the middle world as a watery environment, the ocean in this story; the inner world, various aspects of the otherworld, of which two are mentioned – Isle of Joy and Isle of Women.
Apart from the Isle of Women, all other aspects of the otherworld are subject to the Wheel of change, and people will flow between life and death in a constant cycle. The fighting going on between the winter and summer archetypes of the material world, also goes on in the otherworld. It may be worth viewing the otherworld as having a dreamlike watery quality, where things shape shift like as happens in a dream; it could if you die you will fall into a dream, and then wake up at some point back in a new body in the material world.
An interesting idea is how the Celts view the material world, they describe Ireland as one of many such worlds, of which some are three times larger. Life on other planets? Well this won’t be a problem to the Celt, as their philosophy includes that possibility.
The Isle of Joy is another interesting concept, if the dead set foot on this isle they lose their speech, their intelligence, their minds. The denizens of the Isle of Joy are reduced to watchful laughing morons. In the Bran and Branwen story when the dead were put into the Cauldron of Rebirth they were reborn without the ability to speak. My observation is that babies are watchful, laughing and unable to speak. The Isle of Joy also appears to be equivalent to a type of turning castle in various legends, one aspect that is of death, a trap; the other aspect is the site of the Holy Grail, symbol of rebirth and renewal.
Lastly, the only location that defies the wheel is the Isle of Women, which is described in terms of how a Celt would view their version of paradise. There appears to be a second wheel in the Celtic philosophy symbolised as the moon, which is able to defy the solar wheel, one that is overseen by women. It appears in Celtic philosophy it was the women who dealt with matters of death, which was probably one of the functions of female Druids. Nine is a favoured number amongst the Celts for such women, which may determine how many female Druids served a particular holy site, which often was situated on an island.