a struggle between the Uí Briain and the Uí Cheinnselaig. The contemporary leader of the latter kindred was Donnchad's principal opponent, Diarmait mac Maíl na mBó, King of Leinster. Whilst the Uí Briain certainly allied themselves to Cacht and Echmarcach, Diarmait appears to have backed the descendants of Amlaíb Cuarán, a man whose family appear to have opposed Echmarcach at a latter date. In 1036, Echmarcach replaced Amlaíb Cuarán's son, Sitriuc mac Amlaíb, as King of Dublin. The Annals of Tigernach specifies that Sitriuc fled overseas as Echmarcach took control. An alliance with Donnchad could explain Echmarcach's success in seizing the kingship from Sitriuc. Although Donnchad and Sitriuc were maternal half-brothers—as both descended from Gormlaith ingen Murchada—Donnchad's hostility towards Sitriuc is demonstrated by the record of a successful attack he led upon the Dubliners in 1026. Another factor behind Echmarcach's actions against Sitriuc could concern Knútr. Echmarcach's seizure of Dublin occurred only a year after the latter's death in 1035. There appears to be numismatic evidence, annalistic evidence, and charter evidence indicating that Knútr and Sitriuc had cooperated together in terms of trade and military operations in Wales. In contrast to this apparent congeniality, the relationship between Knútr and Echmarcach appears to have been less amiable. In fact, it is possible that Echmarcach's meeting with Knútr may have bound him from taking action against Sitriuc, and that the confusion caused by Knútr's demise may have enabled Echmarcach to exploit the situation by seizing control of the Irish Sea region. According to a poetic verse composed by the contemporary Icelandic skald Óttarr svarti, Knútr's subjects included Danes, Englishmen, Irishmen, and Islesmen. These Islanders could refer to either the folk of the Isles or Orkney, whilst the Irish seems to refer to the Dubliners. Although the poet's implication that Knútr possessed authority over Sitriuc is not corroborated by any other source, and may therefore be poetic hyperbole, the fact that Sitriuc had been able to undertake a pilgrimage and return home to an intact kingdom in 1028 may demonstrate the extent of influence that Knútr held over the Irish Sea region. This authority, and Sitriuc's apparent close connections with Knútr, could account for the security Sitriuc enjoyed during Knútr's reign. If Echmarcach was a member of the Waterford dynasty, his action against Sitriuc may have been undertaken in the context of continuous dynastic strife between Dublin and Waterford in the tenth- and