Celtic Otherworld in the context of "Silver Branch"

Celtic mythology is the body of myths belonging to the Celtic peoples. Like other Iron Age Europeans, Celtic peoples followed a polytheistic religion, having many gods and goddesses. The mythologies of continental Celtic peoples, such as the Gauls and Celtiberians, did not survive their conquest by the Roman Empire, the loss of their Celtic languages and their subsequent conversion to Christianity. Only remnants are found in Greco-Roman sources and archaeology. Most surviving Celtic mythology belongs to the Insular Celtic peoples (the Gaels of Ireland and Scotland; the Celtic Britons of western Britain and Brittany). They preserved some of their myths in oral lore, which were eventually written down by Christian scribes in the Middle Ages. Irish mythology has the largest written body of myths, followed by Welsh mythology.

The supernatural race called the Tuatha Dé Danann is believed to be based on the main Celtic gods of Ireland, while many Welsh characters belong either to the Plant Dôn ("Children of Dôn") or the Plant Llŷr ("Children of Llŷr"). Some figures in Insular Celtic myth have ancient continental parallels: Irish Lugh and Welsh Lleu are cognate with Lugus, Goibniu and Gofannon with Gobannos, Macán and Mabon with Maponos, and so on. One common figure is the sovereignty goddess, who represents the land and bestows sovereignty on a king by marrying him. The Otherworld is also a common motif, a parallel realm of the supernatural races, which is visited by some mythical heroes. Celtic myth influenced later Arthurian legend.

Annwn ([ˈanʊn]), or Annwfn ([ˈanʊvn]), is the Otherworld in Welsh mythology. Ruled by Arawn (or, in Arthurian literature, by Gwyn ap Nudd), it is a world of delights and eternal youth where disease is absent and food is ever-abundant.

Aos sí (pronounced [iːsˠ ˈʃiː]; English approximation: /iːs ˈʃiː/ eess SHEE; older form: aes sídhe [eːsˠ ˈʃiːə]) is the Irish name for a supernatural race in Gaelic folklore, similar to elves. They are said to descend from the Tuatha Dé Danann or the gods of Irish mythology.

The name aos sí means "folk of the sí"; these are the burial mounds in which they are said to dwell, which are seen as portals to an Otherworld. Such abodes are referred to in English as 'shee', 'fairy mounds', 'elf mounds' or 'hollow hills'. The aos sí interact with humans and the human world. They are variously said to be the ancestors, the spirits of nature, or goddesses and gods.

Irish mythology is the body of myths indigenous to the island of Ireland. It was originally passed down orally in the prehistoric era. In the early medieval era, myths were written down by Christian scribes, who Christianized them to some extent. Irish mythology is the best-preserved branch of Celtic mythology.

The myths are conventionally grouped into 'cycles'. The Mythological Cycle consists of tales and poems about the god-like Tuatha Dé Danann, who are based on Ireland's pagan deities, and other mythical races like the Fomorians. Important works in the cycle are the Lebor Gabála Érenn ("Book of Invasions"), a legendary history of Ireland, the Cath Maige Tuired ("Battle of Moytura"), and the Aided Chlainne Lir ("Children of Lir"). The Ulster Cycle consists of heroic legends relating to the Ulaid, the most important of which is the epic Táin Bó Cúailnge ("Cattle Raid of Cooley"). The Fenian Cycle focuses on the exploits of the mythical hero Finn and his warrior band the Fianna, including the lengthy Acallam na Senórach ("Tales of the Elders"). The Cycles of the Kings comprises legends about historical and semi-historical kings of Ireland (such as Buile Shuibhne, "The Madness of King Sweeny"), and tales about the origins of dynasties and peoples.

In historical Indo-European religion, the concept of an otherworld, also known as an otherside, is reconstructed in comparative mythology. Its name is a calque of orbis alius (Latin for "other world/side"), a term used by Lucan in his description of the Celtic Otherworld.

Comparable religious, mythological or metaphysical concepts, such as a realm of supernatural beings and a realm of the dead, are found in cultures throughout the world. Spirits are thought to travel between worlds or layers of existence in such traditions, usually along an axis such as a giant tree, a tent pole, a river, a rope or mountains.

Samhain (/ˈsɑːwɪn/ SAH-win, /ˈsaʊɪn/ SOW-in; Irish: [ˈsˠəunʲ]; Scottish Gaelic: [ˈs̪ãũ.ɪɲ]) or Sauin (Manx: [ˈsoːɪnʲ]) is a Gaelic festival on 1 November marking the end of the harvest season in autumn and beginning of winter or the "darker half" of the year. It is also the Irish and Scottish Gaelic name for November. Celebrations begin on the evening of 31 October, since the Celtic day began and ended at sunset. This is about halfway between the autumnal equinox and winter solstice. It is one of the four Gaelic seasonal festivals along with Imbolc, Bealtaine, and Lughnasa. Historically it originated in Ireland and it was widely observed throughout Ireland, Scotland, and the Isle of Man. Its Brittonic Celtic equivalent is called Calan Gaeaf in Wales.

Samhain is believed to have Celtic pagan origins, and some Neolithic passage tombs in Great Britain and Ireland are aligned with the sunrise at the time of Samhain. Samhain is mentioned in the earliest Irish literature, from the 9th century, and is associated with many important events in Irish mythology. The early literature says Samhain was marked by great gatherings and feasts and was when the ancient burial mounds were open, which were seen as portals to the Otherworld. Some of the medieval tales also associate Samhain with bonfires and sacrifice.

The Tuatha Dé Danann (Irish: [ˈt̪ˠuə(hə) dʲeː ˈd̪ˠan̪ˠən̪ˠ], usually translated "folk of the goddess Danu"), also known by the earlier name Tuath Dé ("tribe of the gods" or "divine tribe"), are a supernatural race in Irish mythology. Many of them are thought to represent deities of pre-Christian Gaelic Ireland.

The Tuath Dé Danann are often depicted as kings, queens, druids, bards, warriors, heroes, healers and craftsmen who have supernatural powers. They dwell in the Otherworld but interact with humans and the human world. They are associated with the sídhe: prominent ancient burial mounds such as Brú na Bóinne, which are entrances to Otherworld realms. Their traditional rivals are the Fomorians (Fomoire), who might represent the destructive powers of nature, and whom the Tuatha Dé Danann defeat in the Battle of Mag Tuired. Prominent members include the Dagda ("the great god"); The Morrígan ("the great queen" or "phantom queen"); Lugh; Nuada; Aengus; Brigid; Manannán; Dian Cecht the healer; and Goibniu the smith, one of the Trí Dé Dána ("three gods of craft"). Several of the Tuath Dé are cognate with ancient Celtic deities: Lugh with Lugus, Brigit with Brigantia, Nuada with Nodons, Ogma with Ogmios, and Goibniu with Gobannus.

Brú na Bóinne (Irish: [ˈbˠɾˠuː n̪ˠə ˈbˠoːn̠ʲə], "mansion or palace of the Boyne"), also called the Boyne Valley tombs, is an ancient monument complex and ritual landscape in County Meath, Ireland, located in a bend of the River Boyne. It is one of the world's most important Neolithic landscapes, comprising at least ninety monuments including passage tombs, burial mounds, standing stones and enclosures. The site is dominated by the passage tombs of Newgrange (Sí an Bhrú), Knowth (Cnogbha) and Dowth (Dubhadh), built during the 32nd century BC. Together these have the largest assemblage of megalithic art in Europe. The associated archaeological culture is called the "Boyne culture".

Brú na Bóinne is also an important archaeoastronomical site; several of the passage tombs are aligned with the winter solstice and equinoxes. The area continued to be a site of ritual and ceremonial activity in the later Bronze Age and Iron Age. In Irish mythology, the tombs are said to be portals to the Otherworld and dwellings of the deities, particularly The Dagda and his son Aengus. They began to be studied by antiquarians in the 18th century, and archaeological excavations began in the 20th century, when some of the passage tombs underwent restoration.

In Irish mythology, Mag Mell (modern spelling: Magh Meall, meaning 'delightful plain') is one of the names for the Celtic Otherworld, a mythical realm achievable through death and/or glory. Unlike the underworld in some mythologies, Mag Mell was a pleasurable paradise, identified as either an island far to the west of Ireland or a kingdom beneath the ocean. However, Mag Mell was similar to the fields of Elysium in Greek mythology, and similarly was accessible only to a select few. Furthermore, Mag Mell, like the numerous other mystical islands said to be off the coast of Ireland, was never explicitly stated in any surviving mythological account to be an afterlife. Rather, it is usually portrayed as a paradise populated by deities, which is occasionally visited by some adventurous mortals. In its island guise, it was visited by various legendary Irish heroes and monks, forming the basis of the adventure myth or echtrae as defined by Myles Dillon in his book Early Irish Literature. This otherworld is a place where sickness and death do not exist, a place of eternal youth and beauty. Here, music, strength, life, and all pleasurable pursuits come together in a single place. Here, happiness lasts forever, and no one wants for food or drink. It is something of an Irish equivalent to the Valhalla of the Norse as well as the Elysium of the Greeks.

Legends say its ruler is the sea god Manannán mac Lir, or less often the Fomorian King Tethra. Mag Mell's allure extended from the pagan era to Christian times. In later stories, the realm is less an otherworld destination than an earthly paradise which adventurers could reach by traveling west from Ireland, often blown off course by providential tempests while on an inspired mission. They typically explore many other fantastic islands before reaching their destination and returning home (or sailing on). Among these legendary voyagers are St. Brendan, Bran mac Febal (see The Voyage of Bran), and Máel Dúin.