Cycles of the Kings in the context of The Expulsion of the Déisi

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 Fenian Cycle (/ˈfiːniən/), Fianna Cycle or Finn Cycle (Irish: an Fhiannaíocht) is a body of early Irish literature focusing on the exploits of the mythical hero Finn or Fionn mac Cumhaill and his warrior band the Fianna. Sometimes called the Ossianic Cycle /ˌɒʃiˈænɪk/ after its narrator Oisín, it is one of the four groupings of Irish mythology along with the Mythological Cycle, the Ulster Cycle, and the Kings' Cycles. Timewise, the Fenian cycle is the third, between the Ulster and Kings' cycles. The cycle also contains stories about other famous Fianna members, including Diarmuid, Caílte, Oisín's son Oscar, and Fionn's rival Goll mac Morna.