By tradition the dynasty was founded by Conall Corc but named after his ancestor Éogan, the firstborn son of the semi-mythological 3rd-century king Ailill Aulom.

Long, an anglicized version of the name Ó Longaidh, belongs to one of the oldest branches of the Eóghanchta royal dynasty of Ireland's Munster Province.

Prince Longaidh, patriarch of the sept living in about 640, was a descendant of Oengus Mac Nad Fróich, the first Christian king of Munster in the 5th century who was said to have been baptized by Ss.

By the time of the Norman invasion in 1066, this Catholic clan was well established in its present territory in the Barony of Muskerry, County Cork, parishes of Canovee, Moviddy, Kilbonane, Kilmurry, and Dunisky straddling the River Lee.

[3][10] They appear to have initially been subjects of the Dáirine,[citation needed] a warlike people with frequently mentioned connections to Ulster, who were possibly cousins of the Ulaid.

[citation needed] The Dáirine were represented in historical times most clearly by the Corcu Loígde, over whom the Deirgtine finally achieved supremacy during the 7th century, following the loss by the former of their centuries-long hold on the Kingdom of Osraige, apparently with some outside help from the Uí Néill.

Ireland was dominated by several hostile powers whom they were never in any position to challenge militarily on their own, in the early centuries, but there also existed a number of subject tribes whom the Deirgtine successfully convinced to adopt them as their overlords.

The importance of the Cashel kingship was primarily ceremonial, and rulers were with the occasional exception not militarily aggressive, although they continually strove for political dominance as far as they could with the province's wealth.

Strong petty kingdoms regarded as subject would receive large payments called rath in return for their acknowledgment of the political supremacy of Cashel, and they would sometimes give hostages as well.

[3] The most powerful petty kingdoms exchanged hostages with the King of Cashel, and though subject in some sense (by agreement), they were legally free and capable of terminating the contract.

Though descended from Conall Corc and thus theoretically entitled to hold the kingship, in effect these dynasties were excluded from Cashel politics, a situation which may or may not have been based on geographical realities.

[3][15] This is strange in part because the first two were supposedly descended from Conall Corc and not Dáire Cerbba, but this grouping may be simply meant to illustrate that these were all free tribes in comparison with the rent-paying Déisi.

There are several extinct and/or unconfirmed septs: Eóganachta dynastic surnames include O'Callaghan, MacCarthy, O'Donoghue, MacGillycuddy, O'Keeffe, O'Moriarity, O'Sullivan, among others, many of them of contested origin.

There is also great evidence in the pedigrees and regnal lists of repeated modification, outright fabrication, and unceremonious deletion, at least for the early period (all concerned sources), with some criticisms quite severe,[19] although this is also a problem with Connachta and Laigin material.

and the related Uí Liatháin (Lyons, Gleeson, others), ancient allies of the Eóganachta who may have originally belonged to the Dáirine, although it is also possible they were earlier or peripheral branches of the descendants of Ailill Flann Bec, or of Ailill Aulomm, not involved in the innovative Cashel politics of the descendants of Conall Corc, actual founder of the Eóganachta dynasties.

They paid no obvious tribute but were little involved in the political scene after a period, the terms of the alliance being only that they were expected to support the Eóganachta militarily on "honour related" expeditions outside Munster or in the defence of it.

[3] Against this is the fact that the Uí Fidgenti had their own capital at Dún Eochair in Munster, constructed by the Dáirine several centuries before the rise of Cashel, as described by Geoffrey Keating.

The smaller Dál gCais kingdom proved to have surprising military might, and displaced the increasingly beset Eóganachta, who were suffering also from attacks by the Vikings and the Uí Néill, on the Munster throne during the course of the 10th century.

They would badly rout the FitzGeralds at the Battle of Callann, halting the advance of the Normans into Desmond, and win back many territories briefly held by them.

Curley[25] gives profiles of some twenty current Irish lords, several of them Eóganacht or allied, enjoying varying levels of recognition.

He inserted himself into the pedigree of the Sliocht Cormaic of Dunguile, the senior surviving sept of the MacCarthy dynasty, who still await recognition from the Irish government following the scandal.