Briefly, Religion in Erin
Celtic Paganism
Julius Caesar spoke of the Druids as the educated class of Celts, and said that to learn their ways would take 20 years. This, in part, due to the oral traditions of the Celts and lack of written knowledge.
Druids follow the old religion of Erin which includes many gods, more than a hundred, and believing that every part of the natural world has a spirit. Lugh is the king of the gods, universally the most important among them with some of the more important ones (around Mallow anyway, there is considerable regional variation) being Taranis the thunder god, Cernunnos the stag god, and Anu the mother of the gods who feeds them.
The public holy days follow solar astronomy:
Beltaine, the beginning of summer halfway between the vernal equinox and summer solstice (~May 1) and rituals to protect people, livestock, and crops are performed. The ashes from Beltaine bonfires in particular have protective value.
Lúnasa, the beginning of harvest halfway between summer solstice and autumnal equinox (~August 1) and is observed with the funeral feast and games. The athletic competitions part of the holiday is in memory of Lugh’s adoptive mother Tailtiu who died from exhaustion after clearing the plains of Erin for agriculture.
Samhain, the end of harvest and the start of the “dark half of the year,” halfway between autumnal equinox and winter solstice (~November 1). As during Beltaine, bonfires are burned for protection, and food and drink are sacrificed to appease the aos sí (spirits who can more easily enter the physical world near Samhain and Beltaine) to ensure livestock will survive the winter. It is when livestock are brought in from the summer pastures and some are slaughtered. Children go “guising” from door to door in disguise and a gift in the form of food (often apples or nuts) or coins is given out to them.
Imbolc, the beginning of spring, halfway between winter solstice and vernal equinox (~February 1) and a time of cleaning and divining for the year ahead. On this day Cailleach, the goddess of winter and the wilderness gathers firewood for the rest of the winter. If it is fair weather she will collect much, and the winter will be long, but if the weather is foul, winter will be shorter.
Many other rites occur according to lunar astronomy or in combination of lunar and solar. The most important of these for druids is the Ritual of Mistletoe and Oak which may be performed on the sixth day of the moon. In this, druids in white robes climb an oak on which mistletoe is growing and cut some down. Two white bulls are sacrificed and an elixir is made with their blood and the mistletoe which cures infertility in livestock and is an antidote to poisons.
Christianity in Erin
It has been almost a thousand years since the Bishop Palladius landed at Arklow on the east coast and began spreading Christianity on Erin, but many yet hold on to the old ways of the druids. For most of those who have converted, however, paganism merged with Christianity more than being wholly replaced by it. Some Old Holy Days are still celebrated in similar fashion, though with different names and some new traditions. For example, Imbolc is recast as Saint Brigid’s Day, when St. Brigid crosses are woven and holy wells visited. Likewise, the Lúnasa tradition of cutting the first grain and burying it on top of a hill as a sacrifice, the Christians have recast as a pilgrimage climb.
Those bringing Christianity have attempted to replace and suppress pagan beliefs and practices that are incompatible with the teachings of Christianity—and much of paganism is incompatible with Christian beliefs. In some places, non-Christian religion is outright illegal. As a result there are many, many fewer druids and practicing pagans than there once were. The character classes of cleric and paladin are distinctly Christian. Notably, religious tolerance is not one of their virtues. In fact, the most pious usually consider religious intolerance to be virtuous.