Arthurian Saga. A great body of literature has grown up around the personality of Arthur, the great bulk of it having very little to do with the tale's Celtic provenance. For a discussion of the historical references, see Arthur. In terms of Celtic mythology , Arthur occurs as a character both in Welsh and Irish mythology, with references in the folktales and legends of the other Celtic peoples. Cornwall, for example, while only having a slight claim to producing any early Arthurian literature in its language (seeCornish), claims to have been Arthur's home territory.
In Welsh mythology, the story of "Culhwch and Olwen" is perhaps the earliest known full-fledged Arthurian tale in a Celtic language. This is in the "Independent Native Tales" of the Mabinogion, and although it dates to the early eleventh century, it reflects language and customs from a far earlier date. There are also three late Arthurian tales in the Mabinogion: "The lady of the Fountain," "Peredur, Son of Eefrawg," and "Geraint, Son of Erbin." Arthur also appears in "The Spoils of Annwn," which is a tenth century poem and clearly a prototype for the "Holy Grail" quest.
Interestingly, because of the similarities of Arthur and his "knights" to Fionn Mac Cumhail and the Fianna, Arthur appears in Irish myth as the son of the king of Britain and steals the hounds of Fionn, Bran, and Sceolan. The Fianna pursue Arthur to Britain and recover the hounds and Arthur swears fealty to Fionn. However, in medieval times, Arthurian tales became highly popular in Irish literature and the use of Arthurian characters in Irish narratives and tales was a frequent device. In Eachtra an Mhadra Mhaoil(Adventures of the Crop-Eared Dog), Sir Gawain (in Irish, Sir Bhalbhuaidh) helps the dog, in reality the son of the king of lndia, recover his human shape. This is an Arthurian story composed in Ireland in the fifteenth century. Another Irish addition to the saga, dated from the fifteenth century, is entitled Ceilidhe Iosgaide Leithe(Visit of the Grey-Hammed lady). There are references to the story from other sources that suggest it was a highly popular tale of the time. As well as numerous "home grown" Arthurian tales in Ireland, there were direct translations such as the fifteenth century Lorgaireacht an tSoidhigh Naomhtha(Quest for the Holy Grail),edited by S. Falconer, 1953. There are are least 25 identified Arthurian tales in Irish from the medieval period. However, the Arthurian saga, while popular, never assumed the importance in Irish literature as did the Fenian sagas, the tales of Fionn Mac Cumhail and his Fianna, in which many of the motifs of the Arthurian cycle are preserved in forms closer to the original Celtic.   Arthur has become popular in world literature but not through Celtic eyes. From Geoffrey of Monmouth's Historia, whether basedon a translation from a Celtic language or not, the Norman poet Wace conceived and wrote his Roman de Brut, in which a more medieval and knightly setting was introduced and in which the "Round Table" appears for the first time. Then innovations to the story and its collation into a new narrative form were produced by the French poet Chretien de Troyes {fl. A.D. 1160-1190), who added the idea of courtly love and produced the earliest literary version of the "Holy Grail" legend. Then came the English poet Layamon (fl. A.D. 1198-1207), producing the first of the Arthurian legends to be composed in English, but who effectively mixed some native Celtic folk traditions into his saga. In thirteenth century Germany Gottfried von Strassburg and Wolfram von Eschenbach added to the saga. In the fourteenth century, in England, two more important developments were made with The Alliterative Morte d'Arthur (ca. 1360) and Sir Gawain and the Green Knight(ca.1370). Sir Gawain and the Green Knight is regarded as the greatest single Arthurian legend in Middle English writing and introduces tWo major motifs-the "Beheading Game" and the "Temptation to Adultery." The authors of both pieces are anonymous. However, neither was original and both were borrowed from Celtic mythology. The "Beheading Game" appears in the Red Branch Cycle with Cuchulainn, in the role of Gawain, being invited to behead Uath Mac Imoman during the "Feast of Bricriu." In another version, it is Cu Roi who invites the Ulster warriors to cut off his head on condition that he be allowed to cut their heads off at the appointed time.
The tales involving a "Temptation to Adultery" are many and popular.  In the fifteenth century, Sir Thomas Malory' Le Morte d'Arthur gave the ultimate shape to the saga that William Caxton chose for printing in 1485. In modern times, from Tennyson's Idylls of the King through to tales by William Morris, John Masefield, Mark Twain, and T. H. White's classic The Once and Future King (1958), Arthurian stories , have become almost countless, their appeal timeless. It is perhaps ironic that the deeds of a remote Celtic chieftain of the sixth century, fighting to turn back the Anglo-Saxon invasion and the creation of England out of part of Celtic Britain, should have inspired stories that have featured in the literatures of many countries for at least ten decades

Artio. Gaulish goddess, found on a bronze from Berne and probably cognate with Art (Irish for "bear"), because the goddess sits before a huge bear, offering fruit to it.

Artur. [I] A son of Nemed who led his people in battle against the Fomorii at Cramh Ros.

Astrology. As a means of divination, astrology is rarely mentioned in Celtic myth. Yet Pomponius Mela (ca. A.D. 43) referred to the high regard in which the druids were held for their "speculations by the stars." Flavius Magnus Aurelius Cassidorus (ca. A.D. 490-583), mentions a certain Celtic tribe, the Getae, as being learned in natural and moral philosophy and knowing the "course of the twelve signs of the zodiac, and of the planets passing through them and of the whole of astronomy." The Sicilian Greek, Strabo (64B.C.-A.D. 24), spoke of a Celtic druid named Abaris in Athens discussing such matters with the Greeks. At a time when astronomy and astrology were the same science, the Celts were, according to Cicero, Caesar, Pliny, Tacitus, and other classical writers, masters of astronomy.

Athairne the Importunate. [I] A druid and poet who is described as an overbearing satirist from Bed Edar (Howth). He was foster fatherof the poet Amairgen (not the Milesian). Under the laws of hospitality no gift demanded by a poet could be refused. He demanded the eye of the one-eyed King Luain of Connacht. But when he demanded Buan, the wife 'of Mesgora Mac Da Th6, and was refused, he went to Conchobhar Mac Nessa of Ulster and demanded that Ulster make war on Leinster for its affront to the laws of hospitality. In the war Mac Da Th6 is killed.

Ath Liag Fionn. [I] The ford into which Fionn Mac Cumhail threw a flat stone attached to a golden chain that had magical properties. A prophecy said the stone would be found on a Sunday morning that would mark exactly seven years before the world came to an end.

Ath Nurchair. [I] The ford of the Sling Cast in Westmeath. This is where Cet waited in ambush in order to hurl his "brain ball" at Conchobhar Mac Nessa. The slingshot lodged in Conchobhar's forehead. Fingen, his physician, said if the ball was extracted Conchobhar would die. Seven years later Conchobhar fell into a rage, the ball burst in his head, and he died.

Avagddu. [W] See Afagddu.

Avalon. [W] Annwn, the Otherwotld or "Land of the Dead," "Land of Eternal Youth," "The Summer Land," and so on.

Avon Dia. [I] Abhainn Dea, a stretch of river that held back its waves for fear of the mighty duel in the river ford (Ath Ferdia, now Ardee, the ford of Ferdia) between the champions Cuchulainn, and Ferdia.