The Vikings and the monastic community eventually arrived at a form of peaceful coexistence. Indeed the seafaring and trading abilities of the Vikings proved to be a boon to the monastery, which they provided with wine, salt and other commodities. In A.D. 914, there was a massive raid on Cork and Munster from Scandanavia and some historians think that members of this raiding party expropriated the Viking community that already existed there.
By the twelfth century, the descendants of the original settlers had intermarried with the native Irish and become known as the Ostmen or Eastmen. They established Cork as an important trading centre and its importance was enhanced with the coming to power in the twelfth century of the MacCarthy's of Desmond who established Cork as their capital. The MacCarthy's built a residence and fortress near Cork. In Latin this fortress was called vetus castellarum. This is an exact translation of the Irish sean dún, or old fort, and may be identified with the present-day Shandon area of Cork. The Ostmen of Cork acknowledged the overlordship of the MacCarthy Kings of Desmond but would appear to have retained some form of autonomy.
The Ostmen built a fortification on the south island in the Lee and this may have served as a template for the wall of Cork, which was built during the Norman era.
Gina Johnson, an archaeologist and an expert on the development of Cork, has described Hiberno-Norse Cork in the following terms "......it is likely that the town consisted of formalised rows of wattle-walled houses fronting one or more streets. The Vikings probably defined the limits of their settlement possibly with a wattle wall or earthen enclosure surrounding the south island". At this time, the north island of the Lee, known as Dungarvan and identified with the area around the present day North Main Street, was regarded as being outside the city itself.
Ostman Cork was not fated to have a long history. The last known leader of Ostman Cork, Gilbert Mac Turgar, was killed in a sea battle near Youghal in 1173. In 1177, the Ostmen of Cork suffered a fate common to many conquered peoples before and since. Their property was confiscated and they were expelled from the city of Cork, when the city was taken by an invading army of warriors, the Normans.
1169 is one of the most famous dates in the history of Ireland. In that year Normans from Wales landed at Bannow Bay in Wexford and began the Norman conquest of Ireland. With their superior military technology and organisation, the Normans made inroads against the Irish and Hiberno-Norse. In 1171, many of the provincial kings took an oath of fealty to Henry II of England, including Dermot MacCarthy, King of Munster and overlord of Ostman Cork. At the Council of Oxford in 1177, Henry II granted the kingdom of Cork to Robert FitzStephen and Milo de Cogan, but he reserved the city of Cork for himself. An army led by FitzStephen and de Cogan arrived at Cork city in 1177 and took the city, thus beginning the Norman era of the history of Cork. Prince John, Lord of Ireland , visited Ireland in 1185 and some time around that date granted a charter to Cork city which made Cork a corporate town with powers of local government. This status has been retained by Cork since that time to the present day.
The Normans constructed a wall on the south island of the Lee in 1182, possibly based on the former Ostman defensive structure. Over time, this wall was extended and the entire medieval city centre became one of the greatest walled towns of Ireland.