I regret that I cannot give a full answer to your question. However, I can confirm that Orkney was not uninhabited when the Vikings arrived. Human habitation in Orkney goes back almost nine millenia, and Orkney is famous for its extensively preserved neolithic ruins, which form the Heart of Neolithic Orkney, one of Scotland's six UNESCO World Heritage Sites^^1:

Some 5,000 years ago, the prehistoric people of the Orkney Islands began building extraordinary monuments out of stone. Each of the four Heart of Neolithic Orkney sites is a masterpiece of Neolithic design and construction in itself. But together they represent one of the richest surviving Neolithic landscapes in Western Europe.

The series of important domestic and ritual monuments gives us incredible insights into the society, skills and spiritual beliefs of the people who built the monuments.

Skara Brae is a domestic settlement whose stone walls, passageways and stone furnishings – including beds and ‘dressers’ – survive to the present day.

Maeshowe, a chambered tomb, is an extraordinary example of Neolithic architectural genius. It was designed so that the light of the setting sun at the winter solstice focuses on the narrow passageway, illuminating the chamber inside.

The Stones of Stenness circle and henge is a very early example of this type of monument. The surviving stones are enormous, standing up to 6m tall.

The Ring of Brodgar is a great stone circle 130m across. Surrounded by a rock cut ditch, it is set in a spectacular natural amphitheatre of lochs and hills.

The Heart of Neolithic Orkney lies in a wider archaeological landscape rich with remains from Neolithic times and many later periods of Orcadian history.

In addition to these sites, there are also the Tomb of the Eagles and the Dwarfie Stane. There are also excellently preserved neolithic settlements at the Knap of Howar, Barnhouse, and the Ness of Brodgar.

So yes - Orkney was inhabited before the Vikings. Indeed, given its very close proximity to and visibility from the Scottish mainland it would be astonishing if it was not.

There are equally extensive, although younger, pre-Viking sites on Shetland, most notably the Broch of Mousa, Jarlshof, and Stanleydale Temple.

*^1: The others are the Antonine Wall, New Lanark, the Old and New Towns of Edinburgh, the Forth Rail Bridge, and St Kilda.