Norse earldom comprising in the 11th century the Orkney and Shetland Islands and Sutherland and Caithness on the Scottish mainland. The origins of the earldom are obscure. According to medieval Icelandic historical traditions, Orkney was conquered in the late 9th century by King Harald Fairhair of Norway, who granted the islands to his ally Rognvald of More as compensation for the death of his son Ivar on the campaign. Independent Irish sources suggest that it was Rognvald himself who conquered the islands at the same time that the Danes captured York (866) much too early for Harald to have had a hand in events. Archaeological evidence generally supports the conclusion that the main period of Scandinavian settlement in Orkney was around the middle of the 9th century. The story of Harald's expedition, if not actually true, certainly does reflect an early interest in Orkney by the Norwegian kings. Although up to the end of the 11th century the earls of Orkney were effectively independent rulers, they did acknowledge Norwegian sovereignty.
Sigurd the Mighty (d. c892) began the expansion of the earldom, conquering Caithness and Sutherland. Considerable Norse settlement in Caithness followed. The maximum expansion of the earldom took place under Sigurd the Stout (r. c895-1014) who brought the Norse settlements in the Hebrides under his control, and Thorfinn the Mighty (R. c1020-1065), who probably conquered Ross (c1030-1035). Thorfinn is also known to have ruled in the Shetland Islands: whether he was the first Orkney earl to do so is unclear, as it is possible they had formerly been rules from Norway. Many Orkney earls, including Thorfinn, found the islands a convenient base for Viking raiding in the Irish Sea area, and Norse pirates such as Svein Asliefarson continued to harass the British coasts into the second half of the 12th century. By this time, the earldom was a declining power. Shetland was brought back under direct rule by Norway in 1195, and SutherlandCaithness and Ross were conquered by the Scots in 1199-1202: the Hebrides had already been lost to the kingdom of Man in the late 11th century. After the union of Kalmar in 1397, Orkney, along with Shetland, came under Danish sovereignty until they were both ceded to Scotland in 1469.
Orkney and Shetland are unique among the areas settled by Scandinavians during the Viking Age in that the native population (the Celtic Picts in this case) became assimilated to Scandinavian culture and language and not vice versa, as happened in other areas. So complete was this assimilation - a sign probably that the Scandinavian settlers were very numerous - that almost all place-names in Orkney, Shetland and even Caithness are of Norse origin. Orkney and Shetland developed their own Norse dialect, known as Norn. By the beginning of the 15th century, however, the Scots dialect of English was being used in official documents and by the mid-18th century Norn had died out. Physical reminders of the Norse age in Orkney and Jarlshof in Shetland have been excavated, and the ruins of fine early Norse churches, built soon after the Christianization of the islands in the 11th century, survive at Orphir and Egilsay in Orkney. Most impressive of all is the early 12th century cathedral of St Magnus (Magnus Erlendsson) at Kirkwall, Orkney, which is, after Durham Cathedral, perhaps the finest Romanesque building in the British Isles.
Notes from Encyclopaedia of the Viking Age by John Haywood. Thames
& Hudson 2000. pp 142 - 143.