Well as well as being a sailor and as someone who stays on the West Coast of Scotland there are two answers to your question.
The Western Isles are all the islands that run down the coast of Scotland which are not attached or near the mainland, this usually means the Hebrides, from Lewis & Harris in the North, right down through North & South Uist to Barra in the South. Tiree and Coll used to be included but not any more.
But that was not what you asked the group of islands that lie between Dumbarton which is about 10 miles from Glasgow on the north bank of the river Clyde and which used to be the Kings of Scotland castle around the 10th to 12th centuary and Northern Ireland, and the only one that I can think about is Rathlin Isle which is about 2 miles of N. Ireland.
The Northern Isles which has been talked about is from Orkney in the South right upto the Shetland Islands to the north. The Shetland Isles were part of Norway until the the 16th and they still celebrate this connection today with "Uphellia" when they burn a Viking longboat in January.
I have included a picture of Tiree with this as it is a lovely small island which is situated about 20 miles S West of Skye and it gets the full force of the Atlantic winds so has no trees.
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