Ancient Irish history and legends have come down to us through history thanks to the diligent chronicling of the early Christian monks. The best record of the rich Celtic mythological tradition is contained in the four cycles drawn up by twelfth century Christian scribes:
The Mythological Cycle
The Ulster Cycle (also known as the Red Branch Cycle)
The Fenian or Fianna Cycle ( Ossianic cycle)
The Kings, or Historical Cycle.
The Cycles were then classified according to the first words in their titles. The tales were then catagorized as either prem-sceil ("chief tales") or fo-sceil ("minor tales"). Several lists exist, and differ in their content. The Celtic bards themselves, however, did not classify the legends in separate cycles.
The Celtic culture was mainly an oral and symbological culture. The first known attempt at an alphabet is the Ogham script, which dates from eighth century or earlier, possibly written by the Druids in Ogham - when Rome was already coming our way. As this writing was primarily done on wood, hardly any traces survive today. Mention is made of vast libraries of Ogham writing usually inscribed on the bark or wands of hazel and aspen. If this is true, then it is possible that the great oral memory of the Irish was thus preserved through the Celtic era and into the Christian, through the tradition of storytelling, and it was from this source that the Monks gathered their colorful tales.
Around the 12th century, the tales and sagas of of Ireland were organized. The early medieval monks rewrote the oral stories in a style that was designed to be read aloud to noble or royal households. When they set themselves the task of constructing a pseudo-history of Ireland, they also recast the ancient myths and legends into a Christian mold. In doing so, they demoted the old gods to mortals, and rewrote the sagas into an almost indecipherable maze of conflicting events.
Fortunately, there are a number of manuscripts which have survived fairly intact, and there are many others not yet translated into English. The Lebor Gabála or "Book of Invasions" is one of two important manuscripts from which our knowledge of Celtic pre-history is derived. The other is the Dinnshenchas (History of Places), a mythological geography of Ireland.