A large body of pseudo-historical narrative, romance, and verse centring on the imagined successive invasions of early Ireland, culminating in the arrival of the semi-divine Tuatha Dé Danann, their champion Lug Lámfhota, their defeat of the Fir Bolg, and their defeat by the mortal Milesians. It is one of the four major cycles of early Irish tradition, along with the Ulster and Fenian Cycles and the Cycle of the Kings. Somewhat awkward today, the phrase ‘Mythological Cycle’ was coined to describe those early stories that, in the absence of a Celtic cosmology, deal most with origins and the discernible remnants of pre-Christian religion; its first usage pre-dates the currency of ‘Celtic mythology’. While many of the events of the Mythological Cycle appear to occur prior to the action of the other cycles, the composition of individual narratives is now dated after many Ulster and some Fenian stories. Manuscripts of Mythological Cycle narratives are found in the oldest codices, the Book of the Dun Cow (before 1106) and the Book of Leinster (12th cent.), with correlatives in the Dindshenchas (12th cent.), but some narratives, such as Oidheadh Chlainne Lir [The Tragic Story of the Children of Lir], do not appear to have been composed until much later.
More than in the other three cycles, the narrative point of view in the Mythological Cycle is accepting of wizardry and magical transformations. Angus Óg, his lover Cáer, and the Children of Lir are all turned into swans, and Étaín (1) becomes a butterfly. Allusion to magic also pervades the fictionalized history of Ireland, synchronized to agree with biblical revelation and known events from ancient world history. The invasion of history as told in the Lebor Gabála was thought to have begun with Cesair, before the biblical Flood, and continued with five waves of invaders, the Partholonians, the Nemedians, the Fir Bolg, the Tuatha Dé Danann, and finally the Milesians. Each had to contend with the hated predatory pirates lurking off the north-west coast, the Fomorians. Despite the text's inventiveness, modern commentators have argued that the various invaders of the Lebor Gabála have correlatives, admittedly shadowy, in the waves of early Celtic peoples settling on the island. In Cath Maige Tuired [The (Second) Battle of Mag Tuired], a text of deep and mysterious resonances, the Tuatha Dé Danann, under the maimed king Nuadu Airgetlám [of the Silver Hand], do battle with the Fomorians, the champion Lug Lámfhota killing the monstrous giant Balor.
Personages most often cited in the Mythological Cycle are firstly Lug Lámfhota, the Dagda, Nuadu Airgetlám, and Míl Espáine, but also: Áine (1), Ana, Angus Óg, Balor, Boand, Bres, Bran mac Febail, Brian (1), Brigit, the Cailleach Bhéirre, Cairbre mac Ethne, Cesair, Clídna, Cridenbél, Dian Cécht, Donn (1), Donn mac Míled, Eochaid Airem, Eochaid mac Eirc, Étaín (1), Étaín Óg, Fintan mac Bóchra, Goibniu, Iuchair, Iucharba, Lir, Macha, Manannán mac Lir, Midir, the Mórrígan, Nemed, Ogma, and Partholón.
See also AISLINGE ÓENGUSO [The Vision of Angus], ALTROM TIGE DÁ MEDAR [The Nurture of the Houses of the Two Milk-Vessels], CATH MAIGE TUIRED [The (Second) Battle of Mag Tuired], LEBOR GABÁLA [Book of Invasions], OIDHEADH CHLAINNE LIR [The Tragic Story of the Children of Lir], OIDHEADH CHLAINNE TUIREANN [The Tragic Story of the Children of Tuireann], and TOCHMARC ÉTAÍNE [The Wooing of Étaín]. All commentary is addressed to individual narratives.