Legend and Myths of the Irish .
The recognition of the medieval literature with the Tuatha Dé Danann, ('People of the Goddess Danu') this may be connected directly to the early pantheon of Celtic deities. In the belief that there are ancient earthen work structures which dot the landscape, it is claimed still to be inhabited by the gods and goddesses. Many stories are told of humans being brought into these hidden palaces at night as guests at wondrous banquets. This may be how we came to know about, "The Irish mythological cycle."".
The Fomorians are a race of demonic giants, who live off the shore of Ireland, they were ancient occupants of Ireland (or sometimes mentioned as a mythical, prehistoric people who raided and pillaged Ireland from the sea). The Firbolgs were the most successful with the Fomorians and they subdued the Fomorians and managed to live peacefully with them. .
The Mythological Cycle begins with the invasions of Ireland, although it is thought to have five invasions, before this time is the coming of Cessair, Legend tells that the first inhabitants of Ireland were led by Cessair, a granddaughter of Noah who fled to Ireland to escape the flood. She believed that a land never inhabited by man (and therefore not tainted by him) would be safe. She, with her father Bith, two other men, and fifty other women sailed to Ireland in three ships. Only one ship made it to Ireland after surviving storms and being lost at sea. Bith died from drowning, and one of the other men soon after. Cessair died from grief, and soon all the other women had died, leaving one man Fionntan as the sole survivor. Fionntan lived for five thousand five hundred years, as a stag, eagle, and salmon at various points in time. This is how we know of the story of Cessair and the other subsequent invaders; Fionntan witnessed them all and eventually related the tale to scribes .