The Irish believed in an Otherworld, which they described sometimes as underground, such as in the Sídhe mounds, and sometimes located on islands in the Western Sea. The Otherworld was variously called Tír na mBeo ("the Land of the Living"), Mag Mell ("Delightful Plain"), and Tír na nÓg ("Land of the Young"), among other names. It was believed to be a country where there was no sickness, old age, or death, where happiness lasted forever, and a hundred years was as one day. It was probably similar to the Elysium of the Greek mythology and both may have a shared origin in ancient Proto-Indo-European religion. In Irish Immrama ("voyage") tales, a beautiful young woman often approaches the hero and sings to him of this happy land. Sometimes she offers him an apple, or the promise of her love in exchange for his assistance in battle. He follows her, and they journey over the sea together and are seen no more. Their journey may take place in a boat of glass, in a chariot or on horseback (usually upon a white horse, as in the case of the goddess Niamh of the Golden Hair). Sometimes the hero returns after what he believes is a short time, only to find that all his companions are dead and he has actually been away for hundreds of years. Sometimes the hero sets out on a quest, and a magic mist descends upon him. He may find himself before an unusual palace and enter to find a warrior or a beautiful woman who makes him welcome. The woman may be the goddess Fand, the warrior may be Manannán mac Lir or Lugh, and after strange adventures the hero may return successfully. However, even in cases where the mortal manages to return to his own time and place, he is forever changed by his contact with the Otherworld.
This conception of the Otherworld is also preserved in the Welsh story of Branwen, daughter of Llyr, which ends with the survivors of the great battle feasting in the presence of the severed head of Bran the Blessed, having forgotten all their suffering and sorrow, and having become unaware of the passage of time. In Irish lore, Donn, a god of the dead, reigned over Tech Duinn ('The House of Donn'), which was seen as existing on or under Bull Island, located off the Beare Peninsula in the southwest of Ireland. It was believed that the newly-dead journeyed to Tech Duinn, either to remain there forever, or perhaps as a starting-point on their journey to the Blessed Isles across the Western Sea. A Welsh corollary to Tech Duinn is Annwfn, ruled by the Otherworld kings Arawn and Gwyn ap Nudd.
Traditionally, much folklore and folk practice is concerned with preventing the intrusion of spirits into this world, or the loss of humans to the Otherworlds, and many charms and taboos exist for protection from these incidents. Some of this is seen in fairy lore, where humans fear the fairies might steal human babies and leave Changelings in their places. However other traditions think more kindly of the fairies and other spirits, and encourage the leaving of offerings for them, such as milk and baked goods, in order to form a treaty or friendship with them.
In the First Branch of the Welsh tales known as the The Mabinogion, entitled Pwyll, Prince of Dyfed, the eponymous prince offends Arawn, ruler of Annwn (the Welsh Other World), by baiting his hunting hounds on a stag that Arawn's dogs had brought down. In recompense he exchanges places with Arawn for a year and defeats Arawn's enemy Hafgan. Meanwhile, Arawn rules Dyfed. During this year, Pwyll does not sleep with Arawn's wife, earning himself gratitude from Arawn. On his return, Pwyll becomes known by the title Pen Annwn, "Head (or Ruler) of Annwn."
The Tuatha had no other choice than to take refuge under the sídhe, a Celtic word which denotes the hills where the long barrows lay and which is used also to name a special kind of fairies in Ireland and in the Scottish Highlands, the daoine sídhe. All through Ireland legends can be heard about Knocks (from the Irish cnoc), hollow hills which are inhabited by extended fairy communities ruled by a King or a Queen. The best known sídhe sites in Ireland are: Knockma, where the throne of Finvarra (King of the fairies of Connaught) is located, Knockany, ruled by Ainé, Queen of Munster, and Newgrange in county Meath, a megalithic building which is associated with the Angus Óg myth.
The sídhe can be found by humans in certain times in the year, especially at Midsummer, when they are seen often dancing under the moonlight.
In Brittany and in Asturias similar myths are kept. In the Asturian mythology there are many stories who describe human encounters with xanas (fairies), which are dancing around one of them, the Xana Mega, the Queen of Fairies. The castro of Altamira is said to hide an enormous underground realm which is ruled by a royal couple, and whose entrance is found someplace on the hill.
Byzantine scholar Procopius of Caesarea described the Otherworld beliefs of the ancient Gauls. He said it was thought that the Land of Dead lay some place west of Great Britain. The Continental Celtic myths told that once the souls of the dead had left their bodies, they traveled to the Northwestern coast of Gaul and took a boat in direction to Britannia. When they had to cross the Channel, the souls went to the homes of the fishermen, and knocked desperately at their doors. The fishermen went then out of their houses and led the dead to their goal in ghostly ships.
There are still remains of those beliefs in the Breton and Galician traditions. In Brittany the name Bag an Noz is used to denote those ships who carry the dead to their goal: Anatole Le Braz describes in his book La légende de la mort chez les Bretons armoricains, the existence of souls' processions which make their way toward coastal places like Laoual, in order to start their last travel from there.
On the northern coast of Galicia is the village of San Andrés de Teixido, where there is a little hermitage consecrated to Saint Andrew, which keeps, according to the legend, his bones. Because his shrine was less popular that Saint James's, the saint was very sad. Jesus comforted him and said: "Do not worry, Andrés, for those who do not visit you in life will surely visit you in death". And it is still said in Galicia "Anyone who does not visit San Andrés de Teixido when he is alive must visit after he is dead". It is thought that the people who did not visit the sanctuary in life will have to do it after life, taking the form of serpents and lizards: because of this, the pilgrims who travel to the hermit take care of not to step on those animals. San Andrés de Teixido is located near Cape Ortegal, which according Tacitus was the place where "heavens, seas and earth end": it was the End of the World.
Some Spanish authors, like Constantino Cabal, have supposed that the Pagan inhabitants of Northwestern Spain believed that this was the starting place of the souls of the dead on their trip to the Other World. In this manner, traditions of Astorga tell us of a Rock of the Souls (identified with San Andrés de Teixido) situated on the Sea of the Dead, that is, the Ocean which surrounds the Northern Coast of Galicia. These traditions still testify the ancient Celtic beliefs in an "Other World" located beyond the Sea.