Mythological Cycle
The first of these sections or cycles is the Mythological Cycle. As the name suggests, it is the most fantastical of the cycles. Little historical documentation or evidence remains to bear testament to these tales, and yet, they set the foundation for all those that follow.
This ancient cycle begins just after the Great Flood of the Bible, about 2300 to 2500 BC. It recounts the story of how Ireland was settled. Prominent among these tales are those depicting the Tuatha Dé Danann, the god-like or supernatural beings who ruled the land for much of its history, until they were defeated by the human Milesians.
Though there was no written source until the eleventh century, when the Lebor Gabála Érenn or the Book of the Taking of Ireland, chronicled the mythical history of the country, the tales were still well known. The Tuatha Dé Danann represented the deities worshiped in the ancient Irish pagan religion, though they were portrayed as merely god-like or supernatural in the text above, likely due to the shift from paganism to the Christian beliefs and values being embraced by the eleventh century.
As these values grew, the Tuatha Dé Danann became more recognizable as the aos sidhe, the people of the mounds or the fairy folk, who had been forced below ground when they were defeated by the Milesians about 1700 BC. Tales of their exploits, however, continued to be told, regardless of how fanciful or exaggerated they may have been.
Ulster Cycle
The transition to the next cycle takes place in the first century AD. Originally referred to as the Red Branch Cycle, the Ulster Cycle moved away from supernatural beings to warriors and their adventures. The Red Branch was the royal house of the King of Ulster, Conchobar Mac Nessa, who ruled the Ulaid in eastern Ulster and northern Leinster. He was the uncle of Cu Chulainn, a legendary hero, Ulster warrior, and the main character in the cycle. Many of its tales deal with cattle raids, wars waged among the different ruling houses, or one-on-one combat between its champions.
At a time when wealth was measured by the amount of cattle one possessed, it is no surprise that the most famous tale of this cycle is the Táin Bó Cúailnge or the Cattle Raid of Cooley, which was first documented in the Book of Leinster in the 12fth century. In this tale, Medb, the warrior queen of Connachta, plots to steal Donn Cúailnge, the sacred bull. With his fellow warriors incapacitated, however, our hero Cu Chulainn must defend against the invasion single-handedly.
Fenian Cycle
While there was less magic in the Ulster Cycle than there had been in the Mythological Cycle, it still continued to exist in the form of supernatural feats of strength and cunning, curses, and magical objects. So it was in the Fenian Cycle, which was the third group of stories. Set in the 3rd century AD, it recounts the adventures of Fionn Mac Cumhaill and his band of heroic warriors. It is sometimes referred to as the Ossianic Cycle because many of the latter stories were narrated by Fionn's son, Oisín. The theme in this cycle shifts yet again, this time from the warlike attributes of the Red Branch to the more romantic nature of the Fianna, with tales like those of Oisín and Niamh, or Diarmuid and Gráinne.
The stories in the Fenian Cycle recount the trials and accomplishments of the heroes of the Fianna, the nomadic band of hunter warriors led by Fionn Mac Cumhaill. Not copied down until about the 7th or 8th century, the oral tradition was spread much earlier, perhaps by Oisín himself, since he was not only a warrior of the Fianna but also reputed to be the greatest poet of Irish mythology.
Animal symbolism played an important part in this cycle, as many were renowned for holding the keys to magic and knowledge. One of the best-known tales from this period is the Salmon of Knowledge, which comes from the Boyhood Deeds of Fionn Mac Cumhaill.
Kings' Cycle
As we slowly move from pre-history to a more documented historical era, we arrive at the last cycle in Irish mythology, known as the Kings Cycle or the Historical Cycle. The stories from this period are said to have begun around 200 AD and run up to the 11th centuries, by some accounts. The cycle is a mixture of historical figures and mythical settings, actual events and extraordinary deeds. Their purpose was to sing the praises of good kings and perhaps highlight the faults of bad ones. These are the stories of the early Kings of Ireland, the tales of wonder and heroism. At that time, the prosperity of the people themselves was tied to the courage and wisdom of their king, so it was important to know what attributes a good king would possess. And so, these tales center around the actions of the kings, for good or for bad.
One recurring theme throughout the cycle is the idea of a geas being placed upon the ruler. A geas was a magical vow that forbade or demanded something, and in Irish mythology, the hero could have multiple geasa placed upon him. It could be a blessing or a curse, but either way, breaking it could be dire, and in ancient Ireland, that could mean dishonor, banishment, or even death. So, it was best not to break it. Abiding by it, however, brought good fortune.
In truth, this cycle could best be described as quasi-historical, since little evidence exists concerning many of the characters or the deeds they performed. While it is true that men like Niall of the Nine Hostages and Cormac Mac Airt did exist, some of their deeds and adventures may be strewn with exaggerations and hyperbole. The tales of their exploits were, after all, composed by bards and poets, who drew on myths and narratives to tell their tales. And what a story they told.
Whether historical or simply legendary, that hint of the magical that runs through the earlier cycles continues to appear even here. You recall the Tuatha Dé Danann from the Mythological Cycle? They may have been driven below ground centuries before, but that didn't stop them from making cameo appearances in later cycles. For example, the High King Cormac Mac Airt was said to be so level-headed that Manannan mac Lir, the son of the sea god, gave him the goblet of truth to help him make wise decisions.