The name Cinnéide belonged to Brian Boru's father Cennétig mac Lorcáin, King of Thomond, in the tenth century AD.

According to historian C. Thomas Cairney, the O'Kennedys were one of the chiefly families of the Dal gCais or Dalcassians who were a tribe of the Erainn who were the second wave of Celts to settle in Ireland between about 500 and 100 BC.

Current Y DNA testing proves that there were earlier Irish Kennedy ancestors who were genetically separate from the Dál gCais and their Clare origins.

Some current North Tipperary Kennedys are descended from the Muscraige Tire tribe around Lorrha in the 10th and 11th centuries and have different Y DNA from the Dál gCais.

[citation needed] Placenames such as Coolkennedy and Garrykennedy in Upper Ormond and Killokennedy in Thomond are indicative of their longstanding presence in the region.

The sept split into three branches, the chiefs of which were referred to by their hair colours: don (brown), fionn (blond), and rua (red).

For instance, a branch of the family descended from King Donnchadh, son of Brian Boru, settled in Aherloe in south Tipperary, one section of which had the name Cinneide as a surname.

The Kennedys who settled in Ulster are mostly of Scottish origin from the territories of Galloway and Ayr just across the Irish Sea 20 miles (32 km) away.