Celtic mythology can be divided into three main subgroups of related beliefs.
Before we begin, it is important to keep in mind that the Celtic culture (and its religion(s)) were not as contiguous as the more homogeneous Roman or Greek cultures. Our current understanding holds that each tribe along the vast Celtic area of influence had its own deities, corresponding to the Roman Genius. Of the 300+ known deities, few indeed were commonly honored.
The supreme god of the Celtic pantheon appears to have been the Dagda. This word means the Good God, not good in a moral sense, but good at everything, or all-powerful. The Dagda is a father-figure, a protector of the tribe and the basic Celtic god of whom other male Celtic deities were variants. Celtic gods were largely unspecialised entities, and perhaps we should see them as a clan rather than as a formal pantheon. In a sense, all the Celtic gods and goddesses were like the Greek Apollo, who could never be described as the god of any one thing.
Irish tales depict the Dagda as a figure of immense power, armed with a club and associated with a cauldron. In Dorset there is a famous outline of an ithyphallic giant with a club cut into the chalky soil. While this was probably produced in Roman times, it is very likely that it represents the Dagda. In Gaul, the Dagda appeared in the guise of Sucellos, the striker, equipped with a hammer and cup.
The Dagda's consort was known by various names. The most common of these was the Morrigan (Queen of demons - sometimes spelled Morrigna), but she was also known as Nemain (Panic) and Badb Catha (Raven of Battle). She was said to change into a crow or raven and gloat over the blood on the battlefield. Pronounced as More Ree-an.
Belenus was a more regional deity, who was worshipped mostly in Northern Italy and the Gaulish Mediterranean coast. He was primarily a god of agriculture. A great festival called Beltane was associated with him.
The important position of the god Lug, also known as Lugh, in Celtic religion can be seen from the number of place names in which his name appears. The most famous of these are the cities of Lugdunum (cur: Lyon) and Lugdunum Batavorum (cur: Leiden). He is described in the Celtic myths as a latecomer to the list of deities, and is always described as having the appearance of a young man. His weapons were the throwing-spear and sling and a festival called the Lughnasa was held in his honour.
The Celts also worshipped a number of deities of which we know little more than their names. Among these are the goddess Brigit (or Brigid), the Dagda's daughter; nature goddesses like Tailtu and Macha; and Epona, the horse goddess. Male gods included Cu Roi and Goibniu, the immortal brewer of beer.
Cernunnos (the Horned One) is evidently of great antiquity, but we know little about him. It is probably he who appears on the famous embossed silver bowl found in Gundestrup, Denmark which dates from the 1st or 2nd century BC. The Roman writer Lucan (1st century AD) mentions the gods Taranis, Teutates and Esus, but there is little Celtic evidence that these were important deities.
Some of these gods and goddesses may have been variants of each other; Epona the Gallo-Roman horse goddess, for instance, may well have been a development of the goddess Macha, who was mostly worshipped in Ulster. Polytheistic people rarely cared to keep their pantheons in the neat and tidy order in which later scholars would like to find them.
The early Celts did not build temples in which to worship their deities, but held certain groves (nemeton) of trees to be sacred and worthy to be places of worship. Some trees were considered sacred themselves. The importance of trees in Celtic religion is shown by the fact that the very name of the Eburonian tribe contains a reference to the yew tree, and that names like Mac Cuilinn (son of holly) and Mac Ibar (son of yew) appear in Irish myths. Only in the period of Roman influence did the Celts start to build temples, a custom which they would later pass on to the Germanic tribes that displaced them.
Roman writers insisted that the Celts practiced human sacrifice on a fairly large scale and there is peripheral support for this in Irish sources; however, most of this information is secondhand or hearsay. There are only very few recorded archaeological discoveries which substantiate the sacrificial process and thus most contemporary historians tend to regard human sacrifice as an extremely rare occurrence within Celtic cultures.
There was also a warrior cult that centred on the severed heads of their enemies. The Celts provided their dead with weapons and other accoutrements, which indicates that they believed in an afterlife. Before burial, they also severed the dead person's head and shattered the skull to prevent the ghost from wandering.
No mention of the Celts could fail to include a reference to the druids. These people, who have been much romanticised in recent times, were simply the more or less hereditary class of Shamans that characterised all early Indo-European societies. In other words, they were the equivalent of the Indian Brahmin caste or the Iranian magi, and like them specialised in the practices of magic, sacrifice and augury. They were known to be particularly associated with oak trees and mistletoe; perhaps they used the latter to brew medicines or hallucinogenics. To help understand the meaning, the word druid is often believed to come from the root word meaning "oak". Bards, on the other hand, were those who sang the songs recalling the tribal warriors' deeds of bravery. The Celtic culture was NOT a historical culture - meaing it had no written history. It was a spoken history. Before discarding the notion as indicitive of an easily-forgetful history, keep in mind that, historically, cultures that rely on spoken history tend to be better at spoken records than written cultures. The bards were particularly good at this, it is assumed, because it is easier to remember exact words when put to song. In addition, there may have been a class of "seers" or "prophets". Strabo calls them vates, from a Celtic word meaning "inspired" or "ecstatic". It is therefore quite possible that Celtic society had, in addition to the ritualistic and thaumaturgical religion of the druids, a shamanic element of ecstatic communication with the underworld.