Once a diverse cultural group that extended throughout western Europe and even farther south and east, the Celts are today generally associated with the northwestern locales of Wales, Scotland, Cornwall, Brittany, the Isle of Man, and especially Ireland, where individuals of Celtic ancestry predominate. Linked with the Insular branch of Celtic culture, these areas are contrasted with those of the Continental Celts, who occupied the region known as Gaul (today approximately occupied by modern France). In his Commentaries of the first century B. C., the Roman emperor Julius Caesar, then governor of Gaul, recorded his assessments of Celtic religion, mentioning some of their nearly four hundred appellations for divine beings and other specifics of their culture. As Celtic Gaul was absorbed by the Roman Empire, the continental Celts nevertheless continued to practice their native religion and culture until Christianity took firm hold in the region, as it had in the remainder of Europe, by the fifth century A. D. By this time, however, the newly-converted Christian monks in the Insular north, particularly in Ireland, had undertaken measures to preserve their heritage by transcribing manuscript copies of the ancient tales of Celtic gods and heroes. In their manuscripts, many of which where lost in the ensuing centuries of invasion, these scribes recorded the rich mythology of iron-age Ireland.
Preserved for centuries through oral tradition, the stories in these texts were popularly "rediscovered" in 1760 when Scottish poet James Macpherson published his Fragments of Ancient Poetry. He claimed that the poems were translations of verse composed by the third-century Gaelic poet Ossian (Oisín), but the works were shortly thereafter labeled bogus by scholars. Nevertheless, Macpherson's "translations" stirred considerable popular and scholarly interest in all things Gaelic—a Celtic language of ancient Ireland that has since evolved into modern Irish, Manx, and Scots Gaelic dialects. A little over a century later, in the 1890s, an Anglo-Irish movement known as the Irish Literary Revival—led by poet William Butler Yeats, playwright Lady Isabella Augusta Gregory, and other prominent Irish literary figures of the era—spawned a renewed interest in the subject of Celtic mythology by re-introducing its themes, characters, and culture into modern works of Irish literature written in English.
In the contemporary era, scholars have attempted to reconstruct the culture and history of the Celts through what remains of their mythological literature, but have encountered several barriers. One such obstacle has been the long-standing conflation of ancient Irish myth and history. Until the mid-nineteenth century many scholars tended to view the heroes and gods of Celtic mythology as actual historical figures whose activities had simply been exaggerated or "mythologized" over the centuries. This practice (termed euhemerism) has since been largely forsaken and modern scholars no longer construe such figures as Fionn mac Cumhaill and Conchobar mac Nessa as real men of the past, nor regard the extant mythological texts as having any true historical currency.
Further complications for scholars have appeared as the result of the structure of Celtic culture itself. As Georges Dumézil observed of other Indo-European cultures, Celtic society was generally organized into three parts, consisting of groups of warriors (led by the tribal king), priests, and farmers/freemen. Members of the priestly class, called Druids, acted as intermediaries between ordinary individuals and the gods. Revered for their knowledge and wisdom, the Druids legislated in the spiritual realm and were the bearers of the Celtic religious system—which features an amoral ethic that thoroughly disavows good or evil in individual actions and offers a pervasive belief in metempsychosis, or the transmigration of the soul after death. The historical Druids, like their mythological counterparts who often wielded magical powers and were largely characterized as seers with superhuman clairvoyance, demonstrated an aversion to the written word. Thus, their knowledge was transmitted orally, leaving no written record of the ancient sagas until they were first transcribed in the Christian era.
Despite the scarcity of written texts and a legacy of euhemerism in Celtic studies, scholars have been able to preserve and codify considerable portions of Celtic myth using two key manuscripts, both from the twelfth century—the Lebor Gabála Érenn, literally "The Book of the Taking of Ireland" and the Dindshenchas, sometimes rendered as "The History of Places." The former presents a mythological history of Ireland, while the latter amounts to a mythic geography of the island. The work describes Ireland as a pentarchy, divided into five provinces: in the north Ulster, Connacht in the west, Munster to the southwest, in the southeast Leinster, and Mide (corresponding to modern Meath, site of the mythological Tara, the seat of the ancient Irish High King) in the center. In addition to these geographical distinctions, scholars have also outlined four story cycles in Celtic mythology. These include: 1) The Kings Cycle; 2) The Mythological Cycle; 3) The Fenian Cycle; and 4) The Uliah or Ulster Cycle, also called the "Red Branch" Cycle.
The Kings Cycle focuses on a series of "historical" kings, while the Mythological Cycle is concerned with Ireland's mythic prehistory. Derived in large part from the Lebor Gabála Érenn, the Mythological Cycle describes a series of heroes and peoples who were said to have invaded ancient Ireland. The first of these, Fintan, dates to the time of the Biblical Flood. He was followed by the hero Partholón, who along with his kinsmen battled the demonic Fomoire (one-armed and one-legged monstrosities). The next group of settlers, the Fir Bolg, reputedly divided the island into its five provinces, or cóiceds, and ushered in an era of relative peace. They were later defeated by the Tuatha Dé Danann (literally "The People of the Goddess Danu") at the First Battle of Mag Tuired. Suffering the loss of his arm during the battle—grounds for abdication according to Celtic cultural mores—the Tuatha Dé Danann King Núadu stepped down in favor of Bres (whose father was Fomoire and mother Tuatha). Seven years later the Fomoire and Tuatha collided again, this time under the leadership of Balor and the divine Lug, respectively, in the Second Battle of Mag Tuired. Lug defeated Balor and the Fomoire, but the Tuatha later acquiesced to a final group of invaders, the Sons of Míl (corresponding to the modern Irish), whose King, Eremon, forced the Tuatha Dé Danann underground into the so-called sīdh, or faery mounds, which dot the Irish mythological landscape and serve as entrances to a magical subterranean otherworld.
By far the most popular of the Celtic story cycles, the Fenian Cycle, takes its name from its warrior-hero Fionn mac Cumhaill (sometimes anglicized as Finn MacCool). Considered the "exemplary hero of Ireland," Fionn mac Cumhaill eventually becomes leader of the fiana, an elite band of hunter-warriors. Masterful in battle, Fionn possesses the peculiar ability to see clearly into the future and into the past by sucking his thumb—a power granted him when he touched the Salmon of Knowledge, procured by his tutor, the Druid Finegas. The Fenian canon also includes the famous story of Tóruigheacht Dhiarmada agus Ghráinne, translated as "The Pursuit of Diarmaid and Gráinne," as well as the tales of Fionn's father, Oisín, who plays an important role in the Fenian tale Agallamh na Senórach, "The Colloquy of the Old Men."
The Ulster Cycle features the exploits of the northern King Conchobar mac Nessa and the youthful hero Cú Chulainn. The central story of the cycle, the Táin BóCuailnge, or "The Cattle Raid at Cooley"—sometimes described as the "Irish Iliad"—details an attack on the kingdom of Conchobar mac Nessa by the army of Queen Medb of Connacht in order to steal the famed Brown Bull of Ulster. With all the warriors of Ulster waylaid by supernatural labor pains, Conchobar's nephew Cú Chulainn agrees to fight the attacking forces of Connacht single-handedly, engaging one champion per day. He defeats all of his opponents, but is later slain by trickery. Other important figures in the Ulster Cycle include the Druid Cathbad, King Fergus mac Roich, the troublemaker Bricriu, and the tragic Deirdre, who takes her own life rather than submit to the killer of her lover, Naoise.
Further sources of tales in the canon of Celtic mythology appear in two important early medieval manuscripts: Lebor na hUidre, "The Book of the Dun Cow," and Lebor Laignech, called "The Book of Leinster." The former, though fragmentary, contains thirty-seven mythic stories, while the latter includes complete texts of "The Cattle Raid at Fróech" and "The Cattle Raid at Cooley." Many more stories, culled from various sources, including the fourteenth-century text known as The Yellow Book of Lecan, have appeared in various modern editions and continue to delight and intrigue contemporary readers and scholars.