Seventeen hundred years, according to Irish annalists, separate the first from the second, more than two hundred years separate the second from the beginning of the third.
The third cycle is called Fenian because its legends tell, for the most part, of the great deeds of the Feni or Fianna, who were the militia employed by the High King to support his supremacy, to keep Ireland in order, to defend the country from foreign invasion.

They were, it seems, finally organized by Cormac mac Art, 227 A.D.( ?) the grandson of Conn the Hundred Fighter.

But they had loosely existed before in the time of Conn and his son Art, and like all mercenary bodies of this kind were sometimes at war with the kings who employed them.

Finally, at the battle of Gowra, they and their power were quite destroyed.

Long before this destruction, they were led in the reign of Cormac by Finn the son of Cumhal, and it is around Finn and Oisin the son of Finn, that most of the romances of the Fenian cycle are gathered.