The ancestors of the "MacCarthy" clan were known as the Eóganachta and were the premier clan of Munster. According to traditional sources, the Eóganachta descended from Heber - a younger son of the legendary conqueror Milesius. The traditional founder of the dynasty was Corc, the son of Lughaidh, who is purported to have been alive during the time of St. Patrick. Indeed, his grandson was King Óengus of Munster who was baptized by St. Patrick. So it is typically maintained that the dynasty was established during the 5th century and rose to substantial prominence at the start of the early medieval period.

 

It was during the subsequent centuries that the Eóganachta became established as the clans of MacCarthy, MacGillycuddy, MacAuliffe, and O'Sullivan (just to list a few). Of these, the MacCarthy clan became well regarded as politically savvy and sophisticated rulers, but strong and capable warriors. During the 9th century the MacCarthys began to establish a reputation for being "a great scourge to the Danes" (O'Hart, Irish Pedigrees, 1892) and several of the Kings of Munster were especially noted as having successfully warded off the Viking invaders.