Military history of Ireland
The Vikings pillaged monasteries on Ireland's west coast in 795, and then spread out to cover the rest of the coastline. The north and east of the island were most affected. During the first 40 years, the raids were conducted by small, mobile Viking groups. From 830 on, the groups consisted of large fleets of Viking ships. From 840, the Vikings began establishing permanent bases at the coasts. Dublin was the most significant settlement in the long term. The Irish became accustomed to the Viking presence and culture and in many cases became allies, with intermarriage common throughout Ireland. A major Irish battle involving Vikings was fought at the Battle of Clontarf in 1014, in which a large force of Vikings and their Irish allies were defeated by the gathered Irish forces of Brian Boru, then the High King of Ireland along with a small contingent of Viking defectors. After this date Viking activity was sporadic with various Kings and nobles attempting to extend their power base. In 1099, Magnus Barefoot, the King of Norway, and his Viking army had successfully campaigned alongside their Irish allies against opposing Irish warbands. On his return his force was ambushed and killed on the north coast of Ireland.
History of Normandy
The first Viking raids began between 790 and 800 on the coasts of western France. Several coastal areas were lost during the reign of Louis the Pious (814–840). The incursions in 841 caused severe damage to Rouen and Jumièges. The Viking attackers sought to capture the treasures stored at monasteries - easy prey considering the helplessness of the monks to defend themselves. An expedition in 845 went up the Seine and reached Paris. The raids took place primarily in the summers, the Vikings spending the winters in Scandinavia.