From: http://www.av.qnet.com/~raven/myths2.html#The Fianna story
Scota was thought to be the earliest ancestor of the Scots. According to one story, she was the daughter of an Egyptian pharaoh. A wise teacher by the name of Niul, had settled in Egypt and become her husband. They had one child named Goidel, who gave his name to the Gaels. In another story, she was the wife of Milesius and was killed fighting the Tuatha De Danann.
The Sangreal, or Holy Grail, was the holy vessel of Authorian mythology. It was said to be the cup that Christ drank out of at the Last Supper and is believed to have caught the blood which flowed from the spear thrust in His side at the Crucifixion. It was thought to have been brought to Britain by Joseph of Arimathea, the rich man who buried Christ. It may have also been brought by his brother-in-law Bron and his son Alan.
Originally it was Sir Percival who was the first to see the Grail and in later versions of the story, it was changed to Sir Galahad, as the only knight who was worthy enough to see such a vision.
The Grail was somehow lost, but many believed it was hidden somewhere in Britain, as a punishment for all the sinfulness at that time. The mere presence of the holy Grail inspired most of the knights to pursue a path of goodness and virtue. On its arrival at Camelot, the vessel filled King Arthur's hall with the most tasty smells and the knights ate and drank as never before. Sir Galahad was the one who drank from the Grail, as requested by Joseph of Arimathea, which ensured his spiritual survival and was strongly believed that Sir Galahad lives on in a Christian otherworld.
Avalon was another name for the Welsh otherworld, Annwn. Its name suggests it was an island filled with apples. This is where the mortally wounded King Arthur was ferried to, by three mysterious women in a black boat, after the terrible battle against Sir Modred's army. It is said that the undead king will return from Avalon and lead the oppressed Celtic population of Britain to victory over their oppressors. According to one story, Avalon is where Excalibur was forged. Traditionally, Avalon has been associated with Glastonbury, the supposed site of Arthur's tomb.
The Banshee (bean sidhe -- woman of the fairies) is the traditional fairy of the Irish countryside. It was believed that the banshee would attach herself to family and her eerie wail warns them of impending death. They lived underground in sidhe (the fairy heavens), hidden beneath the grassy mounds on the Irish hillsides.
Tuatha De Danann
The Tuatha De Danann are called 'the people of the goddess Dana' in Celtic mythology and were known to most of the Celtic peoples. They were the last generation of gods and goddesses to rule Ireland just before the last invasion of Milesius, who were the ancestors of the present day Irish. The Fomorii were the previous rulers of Ireland, until the Tuatha De Danann defeated them after the second and last battle of Magh Tulreadh. The main reason they won the second battle, was due to their superior magical abilities. Also the Tuatha De Danann were great masters of magic, crafts, and knowledge. They brought with them four talismans: the Stone of Fal, which would scream out loud whenever the true king of Ireland placed his foot upon it; the Magic Sword of Nuada, which was a weapon that only inflicted fatal blows; the spear or sling-shot of the sun god Lugh, and the cauldron belonging to Dagda, who was the father of the gods. It was an inexhaustible pot and was capable of satisfying everyone's appetite.
With the advent of Christianity in Ireland, the Tuatha De Danann did not entirely disappear. In fact, many of their exploits were recorded by the monks, who wrote down many of the Celtic legends. The Tuatha De Danann eventually took up residence underground and became known as the fairies. It is believed, that on the ancient Celtic feast of Samhain, which is celebrated on the last day of October, the Tuatha De Danann would sometimes allow certain mortals to enter their realm.
The Fomorii were the sea gods in Irish myths, who were extremely violent and misshapen. They are described as having only one eye, one hand, or one foot. It is said that the Fomorii emerged from the waves of the ocean and challenged the rulers of Ireland, the Firbolg and the Tuatha De Danann. It was the Tuatha De Danann, who defeated them in battle.
Annwn was a Welsh otherworld, which was a place of peace and plenty. In Annwn there was the fountain of sweet wine and the cauldron of rebirth. The lord of Annwn was the grey-clad Arawn, with whom Pwyll had agreed to exchange shapes and responsibilities for a year. Arawn had a pack of hounds, which were called, 'the hounds of hell.' It is believed that they would fly at night in search of human souls.
Ceridwen was the Welsh goddess of fertility. She was the mother of Afagddu, who was the ugliest man in the whole world. To try and compensate for his ugliness, Ceridwen boiled a cauldron of knowledge for a year and a day, so Afagddu could become the wisest of all and thus be respected. She told Gwion Bach it would be his duty to tend to the cauldron. However Afagddu was denied the prophetic gift, when a drop of the liquid fell on Gwion Bach's finger and unthinkingly he stuck it in his mouth and sucked it off. Furious, Ceridwen hunted him down and eventually ate him. Later however, he was reincarnated as Taliesin, who became the greatest of all the Welsh bards.
Ceridwen had another ugly son, who was called, Morfan. He was a fearsome warrior and fought with King Arthur in his last battle at Camlan. At first none of Sir Modred's men would dare fight against Morfan, because they thought he was so ugly, that surly he must be a devil.
Morrigan is sometimes called Morrigu. She was an Irish goddess of death on the battlefield. She helped the Tuatha De Danann at both battles of Magh Tuireadh. She was also associated with the other war deities: Macha, Badb, and Nemain. Her favorite form to turn into, was that of a crow and as such she settled in triumph and revenge on the shoulder of the Ulster hero Cuchulainn, when he was killed in the war against Queen Medb's forces. Not only had he refused Morrigan's love, but in anger had even wounded her.
Dian Cecht was the Irish god of healing. It's said that he and his daughter Airmid, tended the spring whose waters could restore the dying gods back to life. After the leader of Tuatha De Danann, Nuada, lost his hand fighting in the first battle Magh Tuireadh, Dian Cecht gave him a silver hand, earning him the title, Nuada of the Silver Hand. Even though the Tuatha De Danann were highly impressed by the god's handiwork, they felt that perhaps Nuada was no longer physically able to be their war leader. That was when Bres, who was half Fomorii, took his place. He turned out to be a tyrant and became very unpopular with the Tuatha De Danann. They then decided to restore Nuada to leadership, after Miach, the son of Dian Cecht, had made him a new hand of flesh and blood. Unfortunately, the god of healing became jealous of his son's medical skills and killed him.
Dylan was the Welsh sea god. His parents were Arianrhod and her brother, Gwydion. Soon after he was born, he went straight to the sea and jumped in. Immediately Dylan could swam just as well as the fish. It's said that when his uncle, the smith god Govannon, killed him, all the waves of Britain and Ireland mourned his death.
Cliodhna of Irish mythology, was one of the otherworld goddesses of beauty. It was said that she possessed three very fine, magical birds which could sweetly sing the sick into a deep sleep and cure them. The goddess was passionately in love with a mortal man named Ciabhan, a youth with curling locks of hair. One day, when Ciabhan was hunting and Cliodhna was walking along the shore near Cork, the sea god Manannan Mac Lir put her into a magic sleep and then sent a wave, which pulled her back to the Land of Promise.
The Fianna were the famous warriors responsible for the safety of the High King of Ireland. They were frequently called the Fenians and their leader was Finn Maccool. Most of them came from one of two clans, the Bascna and the Morna. Many of their exploits are related in the tales of the Knights of the Round Table.
To join, "no man was taken till the gound a hole had been made, such as would reach the waist, and he put into it with his shield and a forarm's length of hazel stick. Then must nine warriors, having nine spears, with a ten furrows' width between them and him, assail him and let fly at him. If he sustained injury, he was not received into the band."
Caer was a beautiful fairy maiden, who was loved by the Irish love god, Aonghus. Her father Ethal was one of the Tuatha De Danann. The love god saw Caer in a dream and was so attracted to her beauty, that he fell into a deep sickness. When he discovered who Caer was, he immediately asked her father for her hand in marriage. Ethal however, stated that it was not in his power to grant this because his daughter had taken on the form of a swan. Ethal told Aonghus that he could ask Caer to marriy him only if he was able to recognize her from among the large flock of swans with whom she lived.
When the swans arrived at the Lake of the Dragon's Mouth, the love god immediately recognized Caer and called out her name. Later Aonghus and Caer were married.