The Mythological cycle. . . 

contains the earliest collection of tales in Irish folklore. These legends were handed down from generation to generation over hundreds of years by Filid. The Filid were Bards or Poets and were important people at the time and they were closely connected to the Druids. According to Brehon laws it took a Fili about 12 years to become qualified.
At this time in ancient Ireland everything was handed down orally there were no written documents so it was the Bards job to record all events and these were retold often with the Bards own version. A Bard could shame a high ranking person with the telling of a event which had happened earlier or could raise that persons image to great heights such was the power of the Bard, so how accurate these stories are we cannot say.
I have excluded some sections of the stories because they are just too long but I have included my Bibliography in the source page so further research can be carried out on some of the stories.
It is also worth noting that these stories were written down for the first time with the coming of Christianity in Ireland around 500 AD. While the legends and Myths are based on real events, how much truth there is in todays versions of them can be questioned like the story of Cessair who is believed to have been the first settler in Ireland and a Descendant of Noah . . .
Cessair landed at the Dingle Peninsula in County Kerry. She sailed out with 3 ships but only 1 landed at Dingle Peninsula in County Kerry with 40 women and 3 men on board 1 of the men being Cessair's father Bith who was said to be the son of Noah himself.