Legendary Ireland

There is little scientific knowledge about the people who lived in Ireland between the estimated arrival of humans around 7000 BCE and the arrival of the Celtic Gaels around 100 BCE. The archaeological findings and the marks left by these early inhabitants in the landscape do not reveal their origin. Based on the megalithic monuments and earthworks we assume there was some sort of religion and civilisation on Ireland prior to the Gaels, but until know we have no hard evidence to support any idea about the organisation of pre-Celtic Ireland. By the lack of hard evidence we are forced to built our ideas on myths and legends.

To keep track of the numerous Irish legends they are classified in four so-called cycles. The purpose of this classification is purely academic.
The Mythological Cycle contains the legends referring to the early inhabitants of the island and their deities. The legends of the hero Fionn MacCumhail are collected in the Fenian Cycle. In the Historical Cycle or Cycle of the Kings are the stories regarding legendary and historical, and pre-Christian and Christian kings collected. The forth cycle, the Ulster Cycle, or Cycle of the Red Branch, tells the story of the kingdom of Ulster and its champion Cú Chulain.

Paganism vs. Christianity

Since time immemorial Irish legends has been passed on from generation by generation by oral tradition. It is very likely that the story-lines has been altered in the course of the centuries and perhaps even millennia. Some of these modernisation's are quite obvious. The appearance of the historical third century CE king Cormac MacArt in the much older Finean Cycle for example.
The Ogham alphabet, developed by the Celts, was apparently never used to record legends, so it was not until the Christian era that the legends were put to vellum. Whether or not the assiduous monks and priests have modified the pagan legends to fit in the Christian religion would have depended on their usage of the collected knowledge.

One must assume that Christianisation of Ireland was the ultimate goal of the Church. Just by recording the myths, and therewith demystify the knowledge of the powerful druids, they won the first strike. The most smooth way to achieve large scale conversion was to proof resemblance's between the legends and the Gospels. In order to do so they could have studied the legends to find issues the Gospels could affiliate with or they could twist and bent the legends a bit to fit with the Gospels.
Saint Patrick is known for his ability to use page and Christian symbols. By ousting the snakes from Ireland he did not chased away the cause of the Fall of Man, but the influence of the druids. He also knew what he was doing when he lighted an Easter Fire while a pagan bonfire was burning a few kilometres away. Saint Patrick has proved that knowledge of pagan traditions could be used by the Church, but there is a problem.
The first comprehensive manuscript is the eleventh century Leabhar Gabhála, or in English Book of Conquests, sometimes also called Lebor Gabála Érren, or Book of Invasions of Ireland. Although based on an earlier ninth century manuscript the Book of Conquests is written six centuries after the deeds of Saint Patrick. The Book of Conquests contains a collection of legends referring to the mythological tribes, also known as Lebor Gabal, or the Book of Invasions, but suspicious enough also a copy of the Biblical Book of Genesis and a references to the Tower of Babel, and the following Dispersal of the Nations. Surprisingly there are also links between Biblical and legendary characters and events. It is written for example that one mythological tribe, the Cesair, was led by the granddaughter of Noah and whipped out by the Biblical Flood.

By linking the pagan legends to Biblical events the appearance is against the Church. Unfortunately there is no undelete function in the manuscripts, thus we will never know what the monks and the priests thought to be unfit. However, we have to be careful in judging the writers because some modern-day scholars assume an ancestral link between the last mythical tribe, the Milesians, and the Israelites. So perhaps not all similarities and links are manipulated.