Celtic Mythology

Celtic Mythology (ケルト神話) is the mythology of the Celts, a group of tribal societies present across Europe during the Iron Age and in Medieval times.

In Toaru Majutsu no Index, Celtic Mythology is used as a base for certain forms of Magic.

Principles
There are two main (surviving) strands of Celtic mythology: (Irish/Scots/Manx) and  (Welsh/Cornish/Breton). While they share many traits and have certain figures in common, they do not really overlap; each has its own unique stories. They are further split into  (Ireland) and  (Wales). Mainland Europe's Celtic traditions were mostly lost due to invasion and assimilation of Celtic populations in their conquerors' own societies (mainly the Roman Empire and Germanic tribes). The cultural taboo against consigning knowledge to writing certainly didn't help.

Background
According to the Celts, before people came to the archipelago now called the British Isles, a race of intelligent magical non-humans calling themselves the  ("the children of the goddess ") lived there. With the arrival of people and their permanent settlements, the Tuatha Dé Danann continued to muck about in the lives of people, but retreated to the, their home world, a world still reachable through places such as fairy forts or fairy burrows. This Otherworld is probably the "Fairy Island" Othinus mentions when explaining the Phases.

British Royal Family Arc
During the British Halloween, a spiritual item based on Brionac is used by the Knights of England. The Knight Leader also made use of Hrunting, which incorporated analysis of certain Celtic weapons such as Brionac and Fragarach into its development.

Uses of Celtic Mythology in the story

 * Brionac: A name supposedly ascribed to the spear of the god Lugh.
 * Brionac is a spiritual item used by the Knights of England.
 * Brionac was one of the weapons analyzed by the Knight Leader in his development of Hrunting.
 * Cú Chulainn: A legendary hero of Irish mythology, and son of the god Lugh.
 * The legendary hero is mentioned in the narration when Othinus throws her Gungnir at Kamijou Touma, as apparently having been killed by his own spear as did not return to him unlike the other legendary weapons in mythology.
 * Fragarach: The sword of Manannan mac Lir and Lugh Lamfada. When held at one's throat, the sword was said to prevent them from telling a lie, hence its name, meaning 'Answerer'.
 * Fragarach was one of the weapons analyzed by the Knight Leader in his development of Hrunting.
 * Manannán mac Lir: The Irish god of the Sea, and the psychopomp to five planes of the afterlife.
 * Salome's magic consists of offering weapons as sacrifices to Manannán mac Lir in order to gain their destructive power.
 * Nuada: The first king of the Tuatha Dé Danann, who lost the throne after losing his arm but regained it after gaining a new silver arm, hence his epithet Airgetlám (silver hand/arm).
 * A member of the true Gremlin is called Nuada.
 * Selkie: A legendary being, a that can take the form of a woman.
 * A selkie is depicted in William Orwell's escutcheon as one of its charges.
 * Sheela na Gig: Stone carvings of naked women. It is likely that either the origins of this is from Celtic Mythology or part of an ancient folklorish tradition of the British Isles.
 * Kamijou Touya brought a figure of Shella na Gig to the beach and showed it to Touma, referring to it as a fertility goddess.
 * Tathlum: A ball used by the Celts as a projectile for slingshots, made from the brain and bones of dead enemies hardened with lime.
 * Leivinia Birdway refers to these 'magic balls' during her discussion with Kanzaki Kaori about immortality and the methods that people have used to attempt to reach it.