Every element of improvement or progress which had been introduced into the island disappeared in the long and desperate struggle with the Danes.

The coast-towns which the invaders founded, such as Dublin or Waterford, remained Danish, in blood and manners and at feud with the Celtic tribes around them, though sometimes forced by the fortunes of war to pay tribute and to accept the overlordship of the Irish kings.

It was through these towns however that the intercourse with England which had ceased since the eighth century was to some extent renewed in the eleventh.

Cut off from the Church of the island by national antipathy, the Danish coast-cities applied to the See of Canterbury for the ordination of their bishops, and acknowledged a right of spiritual supervision in Lanfranc and Anselm.

The relations thus formed were drawn closer by a slave-trade between the two countries which the Conqueror and Bishop Wulfstan succeeded for a time in suppressing at Bristol but which appears to have quickly revived.