thumb|300px|The Tuatha Dé Danann in [[John Duncan (painter)|John Duncan's "Riders of the Sidhe" (1911)]]
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 . 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.
Examples of works from the cycle include numerous prose tales, verse texts, as well as pseudo-historical chronicles (primarily the Lebor Gabála Érenn (LGE), commonly called The Book of Invasions) found in medieval vellum manuscripts or later copies. Some of the romances are of later composition and found only in paper manuscripts dating to near-modern times (Cath Maige Tuired and The Fate of the Children of Tuireann).
Near-modern histories such as the Annals of the Four Masters and Geoffrey Keating's History of Ireland (=Seathrún Céitinn, Foras Feasa ar Éirinn) are also sometimes considered viable sources, since they may offer additional insights with their annotated and interpolated reworkings of Lebor Gabála Érenn accounts.
Orally transmitted folk-tales may also be, in a broad sense, considered mythological cycle material, notably, the folk-tales that describe Cian's tryst with Balor's daughter while attempting to recover the bountiful cow Glas Gaibhnenn.
The god-folk of the successive invasions are "euhemerised", i.e., described as having dwelt terrestrially and ruling over Ireland in kingship before the age of mortal men (the Milesians, or their descendants).