By AD 681 the Pictish king, Bridei is said to have devastated Orkney and the Picts seemed to be at odds with other “Scottish” tribes at that time, weakening Pictish power. Thus the Northern Isles were probably in a poorer state by the time the Vikings arrived, bringing their language, culture, religion and trading habits. These were fierce, successful warriors, who had developed sophisticated longships that enabled them at first to successfully raid and eventually colonise coastal communities, and by the late 700s, the Northern Isles were no doubt regularly visited by Vikings. The Anglo Saxon chronicle tells us that Lindisfarne was raided and the monks slaughtered in 793 and similar raids continued over the years until Viking rule was entrenched. Gold and silver had obvious trading uses, but people also were traded: slaves used to run homes and farms, or exchanged as far away as Constantinople.
It is highly likely that Fair Isle’s population was entirely slaughtered, or taken as slaves once the Vikings decided to entrench their sea passage to the West of Scotland. The local people would have been no match for these warriors and there is nowhere to hide or escape. We know from the Orkneyinga Saga that Fair Isle was chosen as a place to site a beacon, probably on Ward Hill. This beacon would have warned of invaders, implying that the island was occupied. It may be that some of the women and children were spared, but since we are unable to distinguish between Pictish genes that existed prior to the invasion, from those that arrived after, we can only guess about who survived. Despite their long residence in the Northern Isles, we continue to know little and surmise a great deal about the Vikings. The much quoted Orkneyinga Saga was written later (about 1200) by an Icelandic writer. Today, there is ongoing discussion about the 'true' history of the Vikings. You can download the report on the conference held in Edinburgh in 2006, where members of the Royal Society of Edinburgh, the Norwegian Academy of Science and Letters and the National Museum of Scotland gathered to discuss current views. (https://www.royalsoced.org.uk/cms/files/events/reports/2005-2006/the_vikings_in_scotland.pdf)
It seems that by 858, when Harald Finehair had united Norway into one kingdom, Shetland was regularly occupied by renegade Vikings, at least during the summer months. In 875 Rognvald Eysteinsson received Orkney and Shetland from Harald as an Earldom, as reparation for his son’s death in Scotland and his brother inherited it from him. By now people lived in longhouses, worshipped pagan gods and adhered to Viking laws. Those who could write, did so in Viking runes. How many of the people in Shetland were of Viking origin at this time, we do not know. It has to be said that there were Viking women warriors, as well as men, so it could be that any remaining local women were simply used at first as servants. Perhaps this situation differed from island to island, depending on the significance of each. We know that Unst, because of its geographical position and outcrops of the much prized steatite, today boasts the remains of up to 60 Viking longhouses, so it must have been a very important site, where men settled to carve the steatite for export, and farmed to survive.
As time passed, Viking culture continued to dominate and any remaining Picts intermarried with Vikings, or newcomers to this successful farming community (once it became a settled entity). By the 11th century, fish, cattle and crops were regularly traded with Norway and elsewhere, although it is likely that some plundering raids probably continued into the 12th century, to augment local coffers, even when Christianity was increasingly adopted. The legacy is in place names, language, customs and genes. Fair Isle’s name at that time was Fridarey, said to mean island of peace, sheep island or fire island (choose your source!)
By the time St Magnus cathedral was begun in Kirkwall in 1137 by Earl Rognvald, Christianity was a firm part of Viking society and people accepted that they were Norwegian. Jon, the last Jarl of the Norse line, died in 1231. Between 1154 and 1472 Orkney was under the control of the Norwegian Archbishop of Trondheim and religious matters in Shetland (and Fair Isle) would be managed from Orkney. Over time the Orkney Earls were all Scots born, holding their positions from the king of Norway in addition to any Scottish lands they owned. Gradually, more Scots moved from the south, attracted in the main to Orkney, and to a lesser degree to Shetland. Norwegian udal law held sway. By 1379 the Sinclairs were the Orkney earls, the last Norse appointment being that of the wealthy Earl William Sinclair.
The Norn language, a dialect of Old Norse, was still spoken for several hundred years, but had died out in most parishes by the 1600s. It remained a living language until about the 1770s in remote places such as Harray in Orkney and Foula in Shetland. We don't know how long it lingered in Fair Isle, but given the island's remoteness, potentially longer than more centrally located districts. As well as fashion and the relative status of the two tongues, the number of Scots speakers in each parish would have differed and helped to drive out Norn at different rates - there is good evidence for more Scots settlement in the South Mainland of Shetland for instance compared to the North Isles.