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.
Viking Age
The first Viking raids began between 790 and 800 along the coasts of western France. They were carried out primarily in the summer, as the Vikings wintered in Scandinavia. Several coastal areas were lost to Francia during the reign of Louis the Pious (814–840). But the Vikings took advantage of the quarrels in the royal family caused after the death of Louis the Pious to settle their first colony in the south-west (Gascony) of the kingdom of Francia, which was more or less abandoned by the Frankish kings after their two defeats at Roncevaux. The incursions in 841 caused severe damage to Rouen and Jumièges. The Viking attackers sought to capture the treasures stored at monasteries, easy prey given the monks' lack of defensive capacity. In 845 an expedition up the Seine reached Paris. The presence of Carolingian "deniers" of "ca" 847, found in 1871 among a hoard at Mullaghboden, County Limerick, where coins were neither minted nor normally used in trade, probably represents booty from the raids of 843–846. After 851, Vikings began to stay in the lower Seine valley for the winter. Twice more in the 860s Vikings rowed to Paris, leaving only when they acquired sufficient loot or bribes from the Carolingian rulers.