The Vikings; Norsemen from Scandinavia; began to raid isolated Irish monasteries in their longboats from the late 8th century onwards. Specifically relevant for Munster were the raids at Inish Cathaigh (816 and 835) and Skellig Michael (824). The raiders chose these monasteries primarily because they were isolated and easy to attack from the Sea; they took provisions, precious goods (metalwork especially), livestock and human captives (these people were either ransomed back if they were high-profile clerics or forced into slavery abroad). In some cases in Ireland, by the mid-9th century, the Vikings set up coastal encampments known as "longphorts"; specifically in relation to Munster, this included; Waterford, Youghal, Cork and Limerick. After first attacking neighbouring Gaelic Irish kingdoms and receiving retribution in return, the mercantile Vikings began to trade with the native Irish and some even intermarried, they also gradually converted to Christianity and eventually became Norse-Gaels, exhibiting elements of both cultures. In Munster itself, a group from among the Vikings; the Uí Ímair, claiming descent from and named for Ivar the Boneless, son of Ragnar Lodbrok; eventually emerged as Kings of small Norse-Gaelic kingdoms where they were Kings of Limerick and Kings of Waterford. These small kingdoms; amongst which Limerick was the most prominent; were involved in rivalries with other Vikings in Ireland and held a complex web of rivalries and alliances with native Irish Gaelic clans. The cultural influence wasn't all one way; some native Irish families in Munster adopted personal names and eventually clan names of Old Norse origin. This includes "Mac Amhlaoibh", with "Amhlaoibh" meaning "Olaf". A prominent example of a Viking-Gaelic alliance in Munster was when the Waterford Vikings joined with Cellachán Caisil, a King of Munster from the Eóganacht Chaisil in 939 against Donnchadh Donn, who was then the High King of Ireland from the southern Uí Néill. The impact of the Vikings, along with pressure from Clann Cholmáin (i.e. the Uí Néill, who dominated the High Kingship of Ireland at the time) led to instability within the Munster Kingship and even permanently broke Osraige from its overkingship. The ascent of elements outside of the main royal families occurred, for instance; St. Cormac mac Cuilennáin from a very much junior branch of the Eóganacht Chaisil became King of Munster during the early 10th century.