The Irish mythological cycle can be divided into four major divisions. The first is the historical-mythological cycle. Two important texts are part of this cycle: the Leabhar Gahbala (Book of Invasions), a mythological history of Ireland; and the Dinnshenchas (History of Places), a mythological geography of Ireland. The main theme in the historical-mythological cycle concerns the peopling of Ireland and the fortunes of the Tuatha De Danann (People of the Goddess Danann), who were the mythological ancestors of the Irish.

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.

The second division is the Ulster cycle. These myths are stories of the warriors of King Conchobar. The themes of those of honor and prestige revolve around heroic deeds and the hero Cu Chulainn (or Cuchulainn).

In the Ulster cycle the heroic accomplishments of Cuchulain are related. Cuchulain in some versions is said to be a foster child of Ulster, and in some respects his character is modeled on that of Lug of the historical-mythological cycle. He is described as a small black-browed man, beardless and full of gaiety. When he is in battle a remarkable change comes over him; he increases in size, and his body trembles and whirls about inside of his skin so that his frontal features are turned to the rear. He can draw one of his eyes back into his head, and his hair bristles on end, with a drop of blood on the end of each hair. When he is in a warrior frenzy he attacks anyone in the vicinity, friend and foe alike.

The third division is that of Fenian. The Fenian Cycle recounts the exploits of Finn Mac Cumhail and his companions and deals with the cult and institution of warriors.

The last division deals with the institution and founding of the great and lesser kings of Ireland.