To understand Celtic literature and the parts of that literature that may represent Celtic mythology, we must have at least a basic understanding of who the Ancient Celts were (and along the way, clear up some misconceptions that are quite prevalent in popular culture these days). First, we must emphatically state that there is not a Celtic ‘race’ – this is a mistaken concept promoted by the Victorians (or earlier), passed along through early 20th century writings, and still (sadly) used by some hate groups today. Being ‘Celtic’ has more to do with language and culture, than it has to do with DNA.
This is not to say that people today are not descended from the Celts (they are!) or that someone does not have Irish ancestry when their grandmother is Irish (they do!). There is not a lone genetic marker for being ‘Celtic’ (although some interesting patterns emerged over the millenia) – and much of the genetic research shows that in many regions we associate with Celtic culture, the primary genetic makeup of the people who live there is the same as those who lived there before Celtic culture arrived or emerged. This is not true everywhere, but it does show that for various reasons the people who were already living in these European regions adopted Celtic language and culture.
Celticism is the result of both culture and language – and occurred in many places in ancient Europe as a result of trade, migration, cultural exchange, and/or battle. Celtic languages and their associated cultures – for a wide variety of reasons – became the predominant set of languages and related culture throughout Europe for many hundreds of years. That fact alone should give us pause when looking to the cultures of the Mediterranean and Near East for the supposed ‘origins’ of our culture (a religocentric and ethnocentric paradigm that is slowly beginning to be revised). We might almost think of it as a cultural worldview, which included a variety of recognizable elements, and was encoded, preserved and transmitted through Celtic languages.

Like the Norse, Romans, Greeks, and other ancient peoples, the Celtic speaking peoples were Indo-Europeans. This simply means that their language (and some elements of culture) can be traced back in time to approximately 5,000 BCE to a group of people living somewhere around the Black Sea (the exact location is somewhat disputed). They began to move around, as ancient people do… some went south and east, and into parts of the East and India, and some north and west into Europe. This is why Sanskrit is related (way way back) to Old Irish, for example… not because of strange migration theories, but because these languages share a parent language, called Indo-European.
Here we must pause to diffuse another popular myth – that the Indo-Europeans were solely violent, patriarchal peoples who violently oppressed previous peaceful, matriarchal cultures. There are many problems with this assertion. First of all, as we saw above in the case of the Celts, in some cases Celtic I.E. language and culture was disseminated through peaceful means, like trade or peaceful migration. Sometimes battle was involved (find me proof of an ancient culture who didn’t at some point use violence, and I’ll eat my sporran) but many social and historical processes were involved in the spread of I.E. language and culture.

Secondly, while a few academics proposed a peaceful, Mother-goddess worshipping culture in some parts of the Near East (something which really has not been proven in any solid way), we can’t just transplant this idea to all of the ancient world (which is exactly what popular writers have done). When those first theories were published, many other scholars yelled out and said, Hey, here are the multitudinous problems with your theory, which show it to be either false, or at least far-fetched and unprovable. The original scholars (yes, Gimbutas is one of them) never stepped up to the plate to respond to these comments and observations. And as a result, a great big conspiracy theory based on false or mythic history has evolved.
This is not to say that we shouldn’t revisit our understanding of history from a female perspective. We absolutely should. And of course we should look at women’s roles, women’s influence on history, and the role of the female divinities (plural) in all of this. This is absolutely crucial, and thankfully it is underway! But this is different from postulating a lost Golden Age that did not in all likelihood exist. This reality does not in any way diminish women’s importance or women ‘s influence then or now, nor does it prevent women in our day and age from focusing on women’s roles and knowledge, and the veneration of (separate) female deities. This is an excellent activity that can help restore balance!….
But, back to the Indo-Europeans: Sometimes by looking at certain linguistic aspects or cultural elements from one Indo-European culture, we are able to shed light on something from another I.E. culture. This is a separate branch of study, and quite complex. But it is one of the tools we have for helping us understand Celtic culture and religion. Here are just a few examples I like to show my students regarding the connections between I.E. languages:

Sanskrit raj (‘king’), Old Irish rige (‘kingship’)… Latin Rex, Gaulish Rix
Old Irish ech (‘horse’), Latin equus, English equine
Gaulish tarwos, Latin taurus, Welsh tarw, Old Irish tarb (‘bull’)
Not all I.E. related words are so visually obvious – I chose these examples because they are – but it is a beginning example of how the languages come from a common parent language, called Proto-Indo-European (P.I.E.) There are also interesting examples of parallels in terms of how society was organized, and some religious symbols and concepts.
Now, of course the ancient Celts did not refer to themselves as indo-Europeans, or even the ancient Celts. In fact, other than one part of ancient Gaul where Caesar alludes to the existence of peoples he calls Celts, we would assume that most Celtic speaking peoples referred to themselves by their tribal names – such as the Parisii (near Paris), Brigantes (in Britain), Cornovii (near Cornwall), Dumnonii (near Devon), Sequani (near the River Seine), etc.
The earliest mention of a word like ‘Celt’ comes from the Greeks, who referred to some of them as Keltoi (showing the hard K sound at the start, as opposed to the ‘s’ sound inexplicably used in the name of Boston’s basketball team). It is possible that tribes other than the one(s) referred to by Caesar used a tribal name similar to Celtii or Celtici or something like this. But not all Celtic speaking people used the word Celt. This does not mean they ‘weren’t Celts’ (a useless and circular argument some academics have taken up, in order to derive assumed authority from a negative and authoritatively pointless stance).

Interestingly, until the last decade, we did not actually know what the word Celt meant! An important paper came out some years back which showed that when the Greeks put the -oi ending on a word it meant ‘Devotee of…’. And the word Celt could be related to a word like Old Irish ceilid (‘hides, conceals’). This would give a meaning of ‘Devotees of the Hidden One.’ Caesar said that some of the Gauls asserted that they were descended from Dis Pater, a relatively minor Roman God associated with the Underworld.
Obviously the Gauls did not mean Dis Pater directly, but in this example of Interpretatio Romana Caesar merely equates the ancestor deity of those Gauls with a deity from his own pantheon who had somewhat similar attributes. The paper goes on to suggest that because of some iconography, the deity in question may have been Cernunnos (an argument I find attractive and convincing).
Other tribes were named for patron gods or goddesses, and even into historical times certain population groups in Ireland and Wales maintained their descent back many generations, ultimately including gods or divine figures in their lineages. Other tribes appear to have named themselves for animals, which may have served as a sort of totem for their tribe, or perhaps was associated with a deity or divine / legendary ancestor. Others took their names from aspects of the landscape or natural world, or attributes they strove to embody. We must be cognizant of both the commonalities and the diversity amongst the Celtic cultures, ancient and modern, in order to approach them with respect, and arrive at any sort of possible known truth.
Sharon Paice MacLeod is a Celticist, author and musician. She is an Old Irish translator at Stanford University and has published several well known books on Celtic religion including ‘Celtic Myth and Religion: A Study of Traditional Belief’ as well as ‘Celtic Cosmology and the Otherworld: Mythic Origins, Sovereignty and Liminality’.