Celtic Religion is a difficult topic to discuss. In large part, it is shrouded in mystery because its tenets were not written down until after those writing had become Christians. Given this circumstance, I will make a few general comments and highlight a few major gods/goddesses based on what seems safe to deduce from the existing records.
In general, the Celts believed that Gods/Goddesses were not all that different from everyday people. In myths Gods/Goddesses are treated much like anyone else. Their treatment of Gods/Goddesses seems related to their belief that the otherworld is not all that different from this one. It is not precisely clear where the otherworld was located, but in many cases it is depicted as a land to the west. In fact, many Celts thought the otherworld could be reached by sailing there.
The Celts also believed that one would need many of the things in the otherworld that one needed this world. Thus, the practice of burying individuals with food, money, weapons, and so forth held sway. The belief that the afterlife was not much different from this life was so strong they even agreed to allow debts incurred in this world to be paid off in the otherworld!
From the forgoing beliefs, It can easily be deduced than that the soul was thought to be eternal. The soul was thought to be able to pass from the otherworld to this one rather easily. In fact, the separation of these two spheres was considered to be almost nonexistent on all hallows eve (Halloween). On that day, ancestors were expected to come calling.
Some authors have argued that the Celts engaged in a widespread practice of human sacrifice. The evidence for this claim is thin. If it occurred, it was most certainly not widespread. The Celts were also accused of being headhunters. They did take the heads of their enemies, ones they admired for one reason or another. However, this was done after a person was killed in battle, not just to collect heads.
Ok, enough of these general observations, lets turn our attention to the major Gods/Goddesses. We restrict our attention to a few of the major figures because there were literally hundreds of Gods/Goddesses most of whom were of local interest.
Cernunnos the horned God depicted in the accompanying graphic was one of the Gods that seemed to be wide spread across Celtic lands (with somewhat different names but with the same characteristic…called Dagda, for instance, in Ireland). Cernunnos was thought to be able to restore life and to be the Lord of Animals (he is horned and often depicted with animals as per the illustration).
Danu (referred to by various names as well…Anu, for instance) is thought to be the goddess of fertility and to have given life to all other gods.
Brigit, the daughter of Cernunnos, is a triple Goddess (most Gods/Goddesses served a tri-part function…. undoubtedly related to the importance of three in Celtic culture…. threes are everywhere ….planting, harvesting, fallow…land, air, sea…mother, daughter, crone….). Brigit was the patroness of poetry, a smithy, and a healer. The knowledgeable reader will note the similarity between this Brigit and St. Brigit as many of the things Brigit accomplished are things also associated with St. Brigit.
Lugus (aka Lugh or Lleu) was a warrior god of shinning light. His influence still survives in a harvest festival known as Lughnasa.
Other important widely spread Gods include Ogmios devoted to eloquence and Camulos a war god.