It is in the history of the settlements formed on the Irish coast by the northern pirates in the ninth century that we must seek for the origin of those relations between England and Ireland which led to an English invasion of the latter country in the reign of Henry II. The earliest intercourse between the two islands had been of a wholly peaceful character; but it had come utterly to an end when Bishop Colman of Lindisfarne sailed back to his old home at Iona after the synod of Whitby in 664. From the hour when her missionary work was done, Ireland sank more and more into the isolation which was a natural consequence of her geographical position, and from which she was only roused at the opening of the ninth century by the coming of the wikings. In the early days of the northmen’s attack upon the British isles it was the tradition of Ireland’s material prosperity and wealth, and the fame of the treasures stored in her religious houses, that chiefly tempted the “white strangers” from the Norwegian fiords across the unknown perils of the western sea; and the settlement of Thorgils in Ulster and those of his fellow-wikings along the eastern and southern coasts of Ireland formed a chief basis for the operations of the northmen upon Britain itself. The desperate fighting of the Irish succeeded in freeing Ulster after Thorgils’s death; but by the middle of the ninth century the wikings were firmly established at four points on the Irish coast, Dublin, Waterford, Cork and Limerick.[369] Under the leadership of Olaf the Fair, Dublin became the head of a confederacy which served as a starting-point and furnished a constant supply of forces for the Danish conquests in England;[370] and for a hundred years afterwards, throughout the struggle of the house of Ælfred for the recovery of the Danelaw, the support given by the Ostmen or wikings of Ireland to their brethren across the channel was at once the main strength of the Northumbrian Danes and the standing difficulty of the English kings.[371] [369] On Thorgils and the wiking settlements in Ireland see _Wars of the Gaedhil with the Gaill_ (Todd), and Green, _Conquest of England_, pp. 66, 67, 74, 76. [370] Green, _Conquest of England_, pp. 90, 91, 107. [371] _Ib._ pp. 213, 242, 252–254, 270–272.