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 Denmark
With the beginning of the Viking Age in the 9th century, the prehistoric period in Denmark ends. The Danish people were among those known as Vikings, during the 8th–11th centuries. Viking explorers first discovered and settled in Iceland in the 9th century, on their way from the Faroe Islands. From there, Greenland and Vinland (probably Newfoundland) were also settled. Utilizing their great skills in shipbuilding and navigation they raided and conquered parts of France and the British Isles.
They also excelled in trading along the coasts and rivers of Europe, running trade routes from Greenland in the north to Constantinople in the south via Russian and Ukrainian rivers, most notably along the River Dnieper and via Kiev, then being the capital of Kiev Rus. The Danish Vikings were most active in Britain, Ireland, France, Spain, Portugal and Italy where they raided, conquered and settled (their earliest settlements included sites in the Danelaw, Ireland and Normandy). The Danelaw encompassed the Northeastern half of what now constitutes England, where Danes settled and Danish law and rule prevailed. Prior to this time, England consisted of approximately seven independent Anglo-Saxon kingdoms. The Danes conquered (terminated) all of these except for the kingdom of Wessex. Alfred the Great, king of Wessex, emerged from these trials as the sole remaining English king, and thereby as the first English Monarch.