According to the BOOK OF CONQUESTS,
the first unnamed inhabitants had perished in the Great Flood.

Then came the children of Partholon,
who waged war against the Fomorians and were finally destroyed by a great pestilence

After them came the people of Nemed,
who fared even worse aginst the Fomorians than their predecessors,
for they were enslaved by them and had to pay every November
a yearly tribute of two thirds of their children and two thirds of their cattle.

At length in a great battle they conquered the Fomorians and killed Conann, their king;
but they themselves were so cruelly disminished in numbers
that they left the country.

Then came the Firbolgs, who had no trouble with the Fomorians,
but were defeated by another wave of invaders,
the Tuatha De Danann.

The Tuatha conquered the Fir Bolgs,
but allowed them to retain the province of Connacht.

They also came into conflict with the Fomorians,
but compromised with them to a certain extent, even to intermarriage.
However, the war broke out again in the end,
and the Fomorians were finally conquered at
the second battle of Moytura.

It has been suggested among the theories of
Fairy Origins that these successive waves of invasion describe the
conflicts of religious cults and practices.
If this is so, the Fomorians would represent a primitive religion that entailed barbaric human and animal sacrifices.
They were a race of sea-pirates with semi-supernatural characteristics
who opposed the earliest settlers in Ireland.

Later claiments identified them with the
Scandinavians, who invaded the island during the eigth century.
TheHighland Fomorians were a race of giants.

In the historical-mythological cycle the story of the
predecessors of the Irish settlement is told.
The first group to come to Ireland is led by a woman, Cesair;
the majority of her group is composed of women.
This group arrives before the great flood,
and all are destroyed in the flood except one, Fintan,
who in the form of a salmon, eagle, or hawk witnesses all of the later settlements.
Fintan is the patron of the traditional lore and storytelling.

The next group is led by Partholan,
but he and all of his people die in a plague.

A third group is led by Nemed;
after suffering many vicissitudes,
this group divides into three parts and abandons Ireland.

Two of these groups, the Fir Bolg (Bolg Men) and the Tuatha De Danann (People of the Goddess Danann),
occupy the subsequent history.
The Fir Bolg return to Ireland, which they divide into the five provinces of
Ulster, Leinster, Munster, Connacht, and Meath;
they also introduce kingship.

When the Tuatha De Danann arrive, warfare ensues over possession of the land.
One tradition states that after the First
Battle of Mag Tuired, the Fir Bolg and Tuatha De Danann make peace and
agree to live together in harmony.

The Tuatha are described as demigods; they are beautiful people,
possessed with skill in music and the arts.
They are always spoken about within a context of fabulous magical powers and wonders,
which define the essence of their manifestation.

A central theme in the myth of the Tuatha is that of the Second Battle of Mag Tuired.

During the First Battle of Mag Tuired the king of the Tuatha, Nuada, is wounded.
Because he is now physically blemished,
he can no longer serve as king.
The kingship is then given to his adopted son, Bres.
Bres's father is a king of the Fomoire,
a group of people with whom Nemed and his people had fought in previous times.
Bres's mother, Eriu, is, however, a Tuatha.
The choice of Bres is apparently an attempt to accomplish an
alliance between the Tuatha and the Fomoire. 
Bres, however, demands severe tribute from the Tuatha and
persecutes them in many ways.

A champion, Lug, arises from among the Tuatha;
Lug is a master of all the arts of magic and warfare.
Meanwhile Nuada, the blemished king, is restored
to his kingship after he has been equipped with a silver arm.
Nuada takes counsel with Lug,
Dagda, the great god with the magic cauldron,
and others concerning the preparations for warfare with the Fomoire.
When the battle finally takes place,
the Tuatha who are slain in the fighting
are magically restored to life.
Lug also uses magic to vanquish Balar of the baleful eye. The Fomoire are routed.
The life of the captured Bres is spared
when he promises to advise on the proper times for sowing and reaping.

; Unlike similar battles in other Indo-European mythologies,
the Second Battle of Mag Tuired does not end in a reconciliation and fusion of the two parties.
The skills imparted by Bres, however, serve the same function
of completing the functions needed in settled society.

The Tuatha are themselves later defeated by the Sons of Mil,
the immediate ancestors of the Irish people.
The Tuatha are said now to live in the underground of Ireland,
in the fairy regions, where the fairies are subject to them.