Scandinavian power in Ireland was now at its height. Large fleets occupied all the lakes in Ulster, so that no part of the surrounding territory was safe from their attacks.[19] The Vikings also retained their grip of the coast towns, and successfully withstood the efforts made by the Irish leaders to dislodge them. Between the years 920 and 950 the importance of Dublin increased considerably through its connection with the Scandinavian Kingdom of Northumbria. Raghnall, grandson of Ivarr, captured York about 919[20] and reigned there until his death in 921.[21] He was succeeded by Sihtric Gale, who had been expelled from Dublin in the preceding year,[22] probably by his brother, Guthfrith. After Sihtric’s death in 927 Guthfrith, King of Dublin (d. 934), with the Vikings of Dundalk, left Ireland in order to secure his own succession in York, but he would seem to have been driven out by Aethelstan, for the Irish Annals mention his return to Dublin after an absence of six months.[23] Guthfrith’s son, Olaf, came forward about this time. Supported by the Norsemen of Strangford Lough he plundered Armagh, but his subsequent attacks on Ulster were checked by Muirchertach MacNeill, son of Niall Glundubh. Olaf fought in alliance with Constantine in the battle of Brunanburh (937), and after the defeat inflicted on them by Aethelstan’s forces he fled to Dublin.[24] He is probably the “Anlaf of Ireland” who was chosen King by the Northumbrians in 941,[25] but he died about a year later.[26] Another Olaf, the famous Olaf Cuaran, also called Sihtricsson to distinguish between them, also played an important part in campaigns in Ireland and England. He went to York about 941, and was elected king by the Northumbrians, but was expelled after a few years along with Raegenald, son of Guthfrith.[27] He then took the Dublin Kingdom under his rule, and in the following year was defeated in battle by the Irish at Slaine (Co. Meath). Leaving his brother Guthfrith to govern in his stead, he departed to York, where he became king a second time; but the Northumbrians drove him out after three years and placed “Yric, son of Harald” (_i.e._, Eric Bloodaxe, late King of Norway) on the throne.[28] Henceforward Olaf limited his activities to Ireland, where he reigned, the most famous of the Dublin Kings, for some thirty years. In 980, having summoned auxiliaries from the Scottish isles and Man, he prepared to attack the _árd-rí_, Maelsechnaill II. A fierce battle was fought between them at Tara in which the Norse armies were completely routed, Olaf’s son Raghnall being among the slain. Maelsechnaill followed up this victory by a three days’ siege of Dublin, after which he carried off a number of hostages from the Norsemen, and also obtained from them 2,000 kine, together with jewels and various other treasures.[29] Olaf himself, utterly disheartened by his defeat, went on pilgrimage to Iona, where he died soon after.