Difference between a Scottish Lowlander and an Scottish Islander?

I can see Rev. Mclean is upset at his son Paul for capitalizing L. In McLean. I've Google it and only found the difference between Highlanders and Lowlanders. (Thinking it was an offensive thing to tge Rev. when the world was much smaller.) Does anybody know the factual difference between Scottish Lowlander & Scottish Islander for that period in time?

google & only have difference in Highlander & Lowlander