The extensive territory of Clann Aeda Buide—Clandeboye—constituted what is now south County Antrim, north and east County Down, and southeast County Derry. It had formed part of the Anglo-Norman earldom of Ulster before the demise of the de Burgh earls in the early fourteenth century. By about 1350, the area had been seized and settled by a branch of the Uf Neill (O’Neills) descended from Aed Buide (Hugh "the yellow-haired"). These were breakaway members of the lineage with aspirations to the kingship, forced to the margins of Tf nEogain (Tyrone) in the decades after Aed’s death in 1283. Within a century the Clandeboye O’Neills had established themselves as one of the most successful uirrithe, or under kings, to emerge in later medieval Ireland. Theoretically vassals of Ua Neill of Tyrone, in reality they were largely autonomous, acknowledging Ua Neill’s claims to overlordship and paying him tribute only by compulsion. By 1450, their power encompassed most of Antrim and Down, and their chieftain, Ua Neill Buide, was reputed a man of great wealth. According to a later English estimate, probably derived from local native sources, Clandeboye was cattle country, its extensive grazing lands capable of feeding many thousands of cows.
Despite the frequent enmity between them, the Clandeboye O’Neills owed their successful settlement of East Ulster to the actions of the O’Neills of Tyrone—particularly to Niall Mor Ua Neill (d. 1398). Initially, by driving out many Anglo-Norman settlers, Niall created the vacuum that the Clandeboye sept was able to exploit. Subsequently, by also waging war on the Scottish MacDonnells, he provided the lineage with ready-made allies willing to help them sustain their struggle against him and his successors. The O’Donnells also supported them, as did, occasionally, the English colonial government in Dublin. Thus, when Eoghan, the Great O’Neill, invaded the territory in 1444, the Clandeboye forces were strong enough to defeat him. A similar attempt by Eoghan’s son, Henry (Enrf) Ua Neill, suffered the same fate in 1476. Efforts to rejuvenate the English colony in 1481 collapsed when Conn Ua Neill of Clandeboye had the government-appointed seneschal of Ulster blinded and castrated.
Clandeboye remained strong until the mid-sixteenth century, when a series of successional disputes weakened it internally, and externally it was menaced first by the territorial ambitions of their erstwhile allies, the MacDonnells, and later by the dramatic reemergence of English military power in East Ulster. After 1584, the English government split the lordship between rival claimants, dividing it into North and South Clandeboye, a development that hastened the family’s decline. Their autonomous status disappeared after the Nine Years’ War, and early in the seventeenth century they lost large parts of their territory.
The valuable manuscript book, the Leabhar Cloinne Aodh Buidhe (Royal Irish Academy, MS 24 p. 33), was composed circa 1680 for the then head of the family, Cormac Ua Neill. It contains some uniquely valuable material of late-medieval provenance, most notably the "Ceart Ui Neill," a list of tributes claimed by the O’Neills throughout Ulster, and a duanaire, or bardic poem book, containing poems by members of the O Gnimh, Mac An Bhaird, O Maolchonaire, and Mac Mhuireadhaigh families.
