a Scandinavian kingdom like the one on the eastern side of the Irish sea. Dublin's ambitions in Ulster were halted by a series of defeats inflicted upon the Norse by Muirchertach mac Néill, the son of Niall Glúndub. According to Benjamin Hudson, "Muirchertach was one of the most successful generals of his day and was described as the 'Hector of the Irish'". In the annals, it is (Duncan) Donnchad Donn from Clann Cholmáin who is titled "high king" after Niall however, and Muirchertach did not succeed his father as king of Ailech either until 938. Apart from his victories over the Norse, Muirchertach led campaigns forcing other provincial kingdoms into submission, most notably taking the king of Munster Cellachán Caisil captive in 941. The same year he led a fleet to the Hebrides, collecting tribute there. When Sihtric died in 927 Gofraid left for York, trying to assume kingship there. He was driven out by Athelstan, and returned to Dublin half a year later. The Vikings of Limerick had taken Dublin in his absence. Gofraid retook the city, but the struggle between Limerick continued well after Gofraid's death in 934. He was succeeded by his son, Amlaíb, who inflicted a decisive defeat on Limerick in 937. The same year Amlaíb went to Northumbria and allied himself with Constantine II of Scotland and Owen I of Strathclyde. Athelstan defeated this coalition at Brunanburh (937), but after Athelstan's death in 939 Amlaíb became king of York. He was joined by a kinsman with the same name, Amlaíb son of Sihtric, known as Amlaíb Cuarán. Congalach mac Máel Mithig, known as Cnogba, succeeded (Duncan) Donnchad Donn as Uí Néill overking in 944 (Muirchertag, who otherwise might have been the obvious successor, had been killed in 943). Congalach was king of Brega and a member of Síl nÁedo Sláine, and the first of this dynasty called "High King" since Cináed mac Írgalaig in the early 8th century. In 944 he sacked Dublin, now ruled by Blácaire mac Gofrith. When Amlaíb Cuaran returned to Ireland the next year, he became ruler of Dublin and acted as an ally of Congalach in the struggle against Ruaidrí ua Canannáin, a rival Uí Néill claimant for High Kingship from Cenél Conaill. This alliance did not last long after Ruaidrí's death in 950, however, and Congalach was killed in 956 in a battle against an alliance of Dublin and