County Cork was the ancient home of the tribes of Érainn, Maritine, Corca Loigde, Muscraige, Uí Liathain, as well as the Éoganacht septs of the Raithlenn and Glendamnacht regions. The Viking settlement of Cork was started around the 9th and 10th centuries. In the 12th century the modern county Cork area was part of the kingdom of Desmhuman and included the territories of Ivelaugh, Beara, Dubh Alla, Insovenagh, Muskerry and Fearmuigh, among others.
Legend attributes the foundation of the monastic settlement to St. Fin Barre, who is said to have begun a monastic school in Cork in 606. Because of the lack of contemporary historical sources for this early period, it is not possible to know if the actual monastic settlement was established as early as this date. However, contemporary records mention the death of the abbot Suibne in 862 and it can be concluded that the monastery must have been well established by this time. The monastery at Cork was a very successful school; it is listed in old Irish sources as one of the most important in the country. The importance of the school at Cork meant that the fame of the settlement, and its patron saint, spread. The site became an important centre for pilgrimage. But although it was ranked among the best in terms of its learning, it never rivalled the other important Irish monasteries of the period - Clonmacnoise, Armagh, Bangor and Kildare - in terms of its size. Throughout its history the settlement seems to have continued on a small scale.
The disruption of this period of Irish monasticism is traditionally attributed to the arrival of Scandinavian raiders in the ninth century. However, the evidence we have from Cork does not support this view. The monastery at Cork, as recorded in the annals, first came under Viking attack in 820. However, in the three and a half centuries following this period there were only three recorded attacks on the monastery. The existence of the reasonably large monastery by the banks of the Lee was probably an important factor in the establishment of a Scandinavian settlement nearby, it seems that the Vikings did not plunder the Cork monastery, but they are more likely to have established mutually beneficial trading arrangements, and the two settlements probably lived peacefully side-by-side. Any problems the Cork monastery had with raiders was as likely to have come from Irish parties as from Viking ones. Irish government was broken up into small tribal lordships and consisted of several warring parties. The overwhelming archaeological evidence for the existence of a Viking town in Dublin is unfortunately not available for Cork. The old city walls were, in several places, built directly on the marsh of Cork. In 848, Olchobar, the king of Cashel, is recorded as having launched an offensive on the Viking dun of Corcaigh. By this date there seems to have been a well established Scandinavian settlement around the Lee.
The Normans had arrived in Cork by 1171. But we have very little historical information for the years 1171-1177. By 1177 however, the Anglo-Normans had begun to colonise and expand the city. They were responsible for its growth into a prosperous medieval town. Extensive trade and contact with the continent meant that the city of Cork was very vulnerable to the spread of the Black Death, when it became rampant in the fourteenth century. The trade in wool was especially relevant to the harbouring of the black rats and the fleas carrying the disease. The Plague hit Cork in 1349, and the consequent fall in population was devastating. It was cited for many years as the reason for the decline of the city of Cork. The decline was on its way anyway, but the plague accentuated it.
The earliest inhabitants of the south-western part of this extensive territory are designated by Ptolemy Uí acute; acute; ni or Uterini, and by other writers Iberni, Iberi, and Juerni. They occupied most of the southern part of the country subsequently called Desmond: their Uí e a Uí itu Uí n prove them to have been of Spanish Iberian origin, and the former, as well as the tribes from which they sprung, and the designation Ibernia or Hibernia, applied to the whole island even by Ptolemy, was derived from the western situation of the country which they inhabited. From Ptolemy's map it appears that the most eastern maritime part of the county in the south of Cork was, in the same age, inhabited by a people whom he called Vodiae or Vodii, but who are unnoticed both by Sir James Ware and Dr. Charles O'Conor. The Coriondi, whose name still bears some affinity to the Irish appellation of this tract, were, according to Smith, the inhabitants of the middle and northern parts, particularly near the present city of Cork, and are said to have sprung from the Coritani, a British tribe occupying a tract of the eastern part of England.
Desmond, signifying "South Munster," was more properly the name of only the south-western part of this principality, which was divided into three portions, of which the whole of that called Ivelagh or Evaugh, and also that called Bear, are included in the modern county of Cork. Bear still retains its ancient name, being divided into the baronies of Bear and Bantry; but Evaugh is included in the barony of West Carberry, which, with East Carberry, Kinalmeaky, and Ibawn or Ibane and Barrymore, anciently formed an extensive territory, deriving its name from its chieftain, Carbry Riada, and in which are said to have been settled four of the eight families of royal extraction of Munster, the head of one of which was McCarty Reagh, sometimes styled prince of Carberry. Kerrycurrihy was anciently called Muskerry Ilane, and comprised also the barony of Imokilly, on the north side of Cork harbour: the only maritime territory remaining unnoticed, viz. Kinnalea, was formerly called Insovenagh. Besides Kerrycurrihy and Imokilly, the entire central part of the county, between the rivers Lee and Blackwater, formed a portion of the ancient territory of Muskerry, which name the western portion of it still retains. The north-western extremity of the county, forming the present barony of Duhallow, is in some old writings calle Uí la and Dub Uí Alla; a Uí ts chief, who, to a very late period, enjoyed almost regal authority, was sometimes styled prince of Duhallow. The remainder, to the north of the Blackwater, formed, before the English conquests, a principality of the O'Keefes, called Fearmuigh.