The ancient Celts saw the Tree of Life as a symbol of balance in the universe. A tree, with roots deep in the earth and branches stretching toward the heavens; the solid trunk of the tree unifying them, both in harmonious balance. The cosmos was shown in the branches and they are woven together showing that everything in the universe is connected. The Tree was a central part of early Celtic spirituality. The most sacred tree of all was the Oak tree which represented the axis mundi, the center of the world. The Celtic name for Oak, “daur”, is the origin of the word door – the root of the oak was literally the doorway to the Otherworld or the Other Side. Manannán mac Lir was the keeper of the gate to the otherworld, the sacred wells were passages to that world. The Otherworld does appear as a specific realm of Life. Rather it is better to accept the term “Otherworld” as being just that: a world other than our own physical world. By this reckoning, the Otherworld implies all of Neamh, all of Muir, and certain non-human parts of Talamh (for remember Talamh is the mortal realm, not just the “earthly” realm of man). Therefore Otherworld is a separate distinction from Talamh, Neamh, and Muir, and it simply refers to a realm outside our normal, human existence.
The Otherworld is called An Sí. It is the supernatural parallel world of Irish, Scottish and Manx mythology and folklore encompassing the three Realms of Sky, Land and Sea. This mysterious realm is sometimes called the Spiritworld but is not the same as what we refers to the Astral Plane. Some might think it imaginary, others might see it as another term for the Collective Unconscious, but Druids believe it is a world to which we sometimes travel in sleep or meditation, and which we enter at the death of our physical body. There, in this parallel universe of the Otherworld, are trees and plants, animals and birds, humans and nature-Spirits. The Otherworld has been described in Irish poetry and tales as being a land of paradise, happiness, and summer. It is often described as a series of islands called “The Fortunate Isles” in the western sea where the various Deities and Ancestors live. Many mythological heroes, such as Cú Chulainn and Brân in The Voyage of Brân, journeyed to Otherworld Realms.
An immram is a class of Old Irish tales concerning a hero’s sea journey to the Otherworld (see Tír na nÓg and Mag Mell). Written in the Christian era and essentially Christian in aspect, they preserve elements of Irish mythology. The immrama are identifiable by their focus on the exploits of the heroes during their search for the Otherworld, located in these cases in the islands far to the west of Ireland. The hero sets out on his voyage for the sake of adventure or to fulfill his destiny, and generally stops on other fantastic islands before reaching his destination. He may or may not be able to return home again. The immrama are generally confused with a similar Irish genre, the Echtrae or “adventure”. Both types of story involve a hero’s journey to an “otherworld”, whether a Christian paradise, a fairyland, the land of the gods or a utopia. They are distinguished by date; echtrai are older, dating from the 7th century, while the earliest immram dates only to the 8th century. David Dumville argues that the immrama include more Christian thinking than the more pagan genre of echtrae and that, whereas the purpose of the echtrai is to enhance understanding of the old gods and the land in which they live, in an immram these pagan elements occur as a challenge to the hero’s faith. In an echtrae the protagonist only ever goes to one location and may arrive in the otherworld with no explanation of the journey, whereas in an immram the hero always has multiple adventures on several islands.
Originally there were seven officially recognized Immram listed in a list of ancient texts. Of those seven only three survive: The Voyage of Mael Dúin, The Voyage of the Uí Chorra, and The Voyage of Snedgus and Mac Riagla. The Voyage of Bran is classified in these same lists as an echtrae, though it also contains the essential elements of the immrama. The later Latin Voyage of St. Brendan also contains a voyage across the sea to various otherworldly islands.