Excerpts from Books and Wikipedia
The Vikings were from Denmark and Norway. Another monastery was attacked the next summer. From then on, the Scandinavians arrived every summer in their longships, raiding the coasts of England, Scotland, and Ireland. [Furtado: 1001 Days] "The Vikings were the most successful fighting men in Europe for much of the ninth, tenth and into the eleventh centuries, for Ireland in the west to Kiev in the east. They were vigorously recruited by the Byzantines." [Cambridge Illustrated History of Warfare, p. 76]