In northern Ireland at this time the head of resistance to the Norsemen was Muirchertach, son of the high-king Niall Glúndubh who fell in the battle of Dublin. A list of his victories is given, a century after his time, by the poet-historian Flann of Monasterboice. Among them is mentioned an expedition by sea against the Norsemen of the Hebrides--it is also mentioned in the genealogies but not in the contemporary annals. The annals on the other hand record that in 939 Muirchertach was captured in Ailech and carried off by the Norsemen to their ships but was immediately ransomed. The event shows that Ailech, one of the great prehistoric stone fortresses, was still occupied in the tenth century by the kings who took their title from it. Especially interesting in Muirchertach's career are his relations with the high king Donnchadh. In the ordinary course of the alternate succession, Muirchertach, as king of Ailech, was the designated successor in the high-kingship to Donnchadh, who was king of Meath. At times he appears prepared to dispute the authority of Donnchadh, at other times he is active in upholding it. His most remarkable action is what is known as his Circuit of Ireland, in 941, briefly noticed in the Annals but described at length in a poem by Cormacán Éces, who accompanied the expedition. With a picked force, said to number 1000, Muirchertach marched through all the principal kingdoms of Ireland, and exacted hostages from each king. In Cashel, he took the king himself, Cellachán, as a hostage. The Dalcassians alone stood off, and after four days marching here and there in their territory, Muirchertach passed on to Connacht without the hostages of Dál gCais. The fact of this expedition illustrates what I have already said, that, from the sixth to the thirteenth century, there was no military organisation in Ireland. The hostages were brought to Ailech and there hospitably entertained by the king and queen for some weeks, after which Muirchertach, so to speak, regularised his position in the matter by handing over all the hostages to the high king Donnchadh. Two years later, in 943, Muirchertach fell in battle with the Norsemen near Dundalk. The high king Donnchadh died in the following year, 944. In the ordinary course of the alternate succession, he should have been succeeded by the king of Ailech, but Muirchertach's death left this kingship either disputed or divided, and the high-kingship was assumed by Congalach, king of Bregia, who reigned for twelve years and fell in battle with the Norsemen. This reign of Congalach is the only breach in the alternate monarchy between the years 734 and 1002.