From: http://www.ireland.org/irl_hist/hist10.htm
It is only recently that we have realised the all important part
played by legendary lore in forming and stamping a nation’s
character. A people’s character and a people’s heritage
of tradition act and react upon each other, down the ages, the
outstanding qualities of both getting ever more and more alike
- so long as their racial traditions are cherished as an intimate
part of their life. Of all the great bodies of ancient Irish
Legendary lore, none other, with the possible exception of the
Red Branch cycle, has had such developing, uplifting, and educational
effect upon the Irish people, through the ages, as the wonderful
body of Fenian tales in both prose and verse, rich in quality
and rich in quantity. Fionn MacCumail, leader of the Fian (Fenians),
in the time of Cormac MacArt, is the great central figure of
these tales. The man Fionn lived and died in the third century
of the Christian Era. It was in the reign of Conn, at the very
end of the second century, that was founded the Fian - a great
standing army of picked and specially trained, daring warriors,
whose duty was to carry out the mandates of the high kin - "To
uphold justice and put down injustice, on the part of the kings
and lords of Ireland - and to guard the harbors from foreign
invaders". From this latter we might conjecture that an
expected Roman invasion first called the Fian into existence.
They prevented robberies, exacted fines and tributes, put down
public enemies and every kind of evil that might afflict the
country. Moreover they moved about from place to place all over
the island. Fionn, being a chieftain himself in his own right,
had a residence on the hill of Allen in Kildare. The Fianna (bodies
of the Fian) recruited at Tara, Uisnech and Taillte fairs. The
greatest discrimination was used in choosing the eligible ones
from amongst the candidate throng - which throng included in
plenty sons of chieftains and princes. Many and hard were the
tests for him who sought to be one of this noble body. One of
the first tests was literary for no candidate was possible who
had not mastered the twelve books of poetry. So skilful must
he be in wood running, and so agile, that in the flight no single
braid of his hair is losed by a hanging branch. His step must
be so light that underfoot he breaks no withered branch. In facing
the greatest odds the weapon must not shake in his hand . When
a candidate had passed these tests and was approved as fit for
his heroic band, there were also vows to be taken as the final
condition of his admission. There were three cathas (battalions)
of the Fian - three thousand in each catha. This was in time
of peace. In time of war the quota was seven cathas. Although
the Fianna were supposed to uphold the power of the Ard Righ,
their oath of fealty was not to him, but to their own chief,
Fionn. The best stories of the Fian are preserved to us in the
poems of Oisin, the son of Fionn, the chief bard of the fian,
in the Agallamh na Seanorach (Colloquy of the Ancients) of olden
time. This is by far the finest collection of Fenian tales, and
is supposed to be an account of the Fian’s great doings,
given in to Patrick by Oisin and Caoilte, another of Fionn’s
trusted lieutenants, more than 150 years after. After the overthrow
of the Fian, in the battle of Gabra in the year 280 A.D.,Caoilte
is supposed to have lived with the Tuatha de Dannann, under the
hills - until the coming of St. Patrick. Oisin had been carried
away to the Land of mortal existence, and to Ireland, when Patrick
is in the land, winning it from Crom Cruach to Christ.