Vikings
Some of the Irish annals, among them the Chronicon Scotorum, record that early in Ólchobar's reign Emly was attacked by a Viking force. In 848, a year which saw multiple defeats for the Vikings, Ólchobar joined forces with his eastern neighbour, Lorcán mac Cellaig, King of Leinster, to defeat a Viking army at Sciath Nechtain, near modern Castledermot, County Kildare. Early sources say two hundred Vikings were killed, later ones increase the number of dead, among them one Tomrair, jarl and deputy of the king of Laithlind. Later in the year the Cashel branch of the Eóganachta inflicted a defeat on Vikings at Dún Maíle Tuile, near Cashel. Further victories had been won in the west, in modern County Sligo, by the High King Máel Sechnaill mac Máele Ruanaid and his ally Tigernach mac Fócartai.
Late in 848 Ólchobar is said to have set up a camp from which the Vikings at Cork were blockaded. The result of this siege is nowhere recorded. This is the first mention of the Vikings at Cork. They are not heard of again until 865. It has been suggested that these several campaigns against Vikings were a coordinated effort by the chief Irish kings.
Read more about this topic: Ólchobar Mac Cináeda