Lebor Gabála Érenn in the context of Book of Leinster

Irish mythology is the body of myths indigenous to the island of Ireland. It was originally passed down orally in the prehistoric era. In the early medieval era, myths were written down by Christian scribes, who Christianized them to some extent. Irish mythology is the best-preserved branch of Celtic mythology.

The myths are conventionally grouped into 'cycles'. The Mythological Cycle consists of tales and poems about the god-like Tuatha Dé Danann, who are based on Ireland's pagan deities, and other mythical races like the Fomorians. Important works in the cycle are the Lebor Gabála Érenn ("Book of Invasions"), a legendary history of Ireland, the Cath Maige Tuired ("Battle of Moytura"), and the Aided Chlainne Lir ("Children of Lir"). The Ulster Cycle consists of heroic legends relating to the Ulaid, the most important of which is the epic Táin Bó Cúailnge ("Cattle Raid of Cooley"). The Fenian Cycle focuses on the exploits of the mythical hero Finn and his warrior band the Fianna, including the lengthy Acallam na Senórach ("Tales of the Elders"). The Cycles of the Kings comprises legends about historical and semi-historical kings of Ireland (such as Buile Shuibhne, "The Madness of King Sweeny"), and tales about the origins of dynasties and peoples.

The Mythological Cycle is a conventional grouping within Irish mythology. It consists of tales and poems about the god-like Tuatha Dé Danann, who are based on Ireland's pagan deities, and other mythical races such as the Fomorians and the Fir Bolg. It is one of the four main story 'cycles' of early Irish myth and legend, along with the Ulster Cycle, the Fianna Cycle and the Cycles of the Kings. The name "Mythological Cycle" seems to have gained currency with Arbois de Jubainville c. 1881–1883. James MacKillop says the term is now "somewhat awkward", and John T. Koch notes it is "potentially misleading, in that the narratives in question represent only a small part of extant Irish mythology". He prefers T Ó Cathasaigh's name, Cycle of the Gods. Important works in the cycle are the Lebor Gabála Érenn ("Book of Invasions"), the Cath Maige Tuired ("Battle of Moytura"), the Aided Chlainne Lir ("Children of Lir") and Tochmarc Étaíne ("The Wooing of Étaín").

The Fomorians or Fomori (Old Irish: Fomóire, Modern Irish: Fomhóraigh / Fomóraigh) are a supernatural race in Irish mythology, who are often portrayed as hostile and monstrous beings. Originally they were said to come from under the sea or the earth. Later, they were portrayed as sea raiders and giants. They are enemies of Ireland's first settlers and opponents of the Tuatha Dé Danann, the other supernatural race in Irish mythology; although some members of the two races have offspring. The Tuath Dé defeat the Fomorians in the Battle of Mag Tuired. This has been likened to other Indo-European myths of a war between gods, such as the Æsir and Vanir in Norse mythology, the Olympians and Titans in Greek mythology, and the Devas and Asuras in Indian mythology.

One theory is that the Fomorians were supernatural beings representing the wild or destructive powers of nature; personifications of chaos, darkness, death, blight and drought.

Conaire Mór (the great), son of Eterscél, was, according to mediaeval Irish legend and historical tradition, a High King of Ireland sometime during the 1st century BC or 1st century AD. His mother was Mess Búachalla, who was either the daughter of Eochu Feidlech and Étaín, or of Eochu Airem and his daughter by Étaín. In the Old Irish saga Togail Bruidne Dá Derga he is conceived when his mother is visited by Nemglan who flies in her skylight in the form of a bird, and is brought up as Eterscél's son.

According to the Lebor Gabála Érenn, he took power after killing his predecessor, and his father's killer, Nuadu Necht. In Togail Bruidne Dá Derga he succeeds Eterscél directly. When Eterscél dies, a bull-feast is held. A bull is killed, a man eats his fill of its meat and drinks its broth, and sleeps as incantations are chanted over him. Whoever this man sees in his sleep will be the new king. He sees a naked man coming along the road to Tara with a stone in his sling. The young Conaire, meanwhile, is hunting birds in his chariot. He chases them into the sea, where they become armed men, announce themselves as Conaire's father's bird troop, and inform him that it is forbidden for him to hunt birds. The leader of the bird troop is called Nemglan and he tells Conaire to go naked to Tara, where he will be made king, and places several geasa (taboos) on his reign. Among other things, he may not be preceded by three red men into the house of a red man. As he approaches Tara he is met by three kings carrying clothes for him, and when he arrives he is made king.

Medieval Irish historical tradition held that Ireland had a High King (Ard Rí) based at Tara since ancient times, and compilations like the 11th-century Lebor Gabála Érenn, followed by Early Modern works like the Annals of the Four Masters and Foras Feasa ar Éirinn, purported to trace the line of High Kings. John T. Koch explains: "Although the kingship of Tara was a special kingship whose occupants had aspirations towards supremacy among the kings of Ireland, in political terms it is unlikely that any king had sufficient authority to dominate the whole island before the 9th century". Máel Sechnaill I is often considered the first historical High King, although he faced some opposition. Applying the title to earlier kings is considered anachronistic, while kings from before the 5th century are generally considered legendary. The traditional list of High Kings is thus a mixture of historical facts and legend.

The annals describe some later High Kings as rígh Érenn co fressabra ("Kings of Ireland with opposition"), which is a reference to the instability of the kingship of Tara from the death of Máel Sechnaill mac Domnaill in 1022. He had been overthrown by Brian Boru in 1002, and restored in 1014 following Brian's death, but Brian's example was followed by many other dynasties in the century following 1022. The High Kingship was effectively ended in the 1170s after the Anglo-Norman invasion, its last holder being Ruaidrí Ua Conchobair.

In Irish mythology, Cian or Cían (Irish pronunciation: [ciənˠ]), nicknamed Scal Balb, was the son of Dian Cecht, the physician of the Tuatha Dé Danann, and best known as the father of Lugh Lamhfada. Cían's brothers were Cu, Cethen, and Miach.

Cían was slain by the Sons of Tuireann, for which Lugh demanded various treasures around the world as éraic (compensation), according to the account in the "Book of Invasions" (Lebor Gabála Érenn, LGE) as well as the late romance version "The Fate of the Children of Tuireann".

Boann or Boand is the Irish goddess of the River Boyne (Bóinn), an important river in Ireland's historical province of Meath. According to the Lebor Gabála Érenn and Táin Bó Fraích she was the sister of Befind and daughter of Delbáeth, son of Elada, of the Tuatha Dé Danann. Her husband is variously Nechtan or Elcmar. With her lover the Dagda, she is the mother of Aengus.