ScotlandsPeople.gov.uk now has searchable registers of lightkeepers employed by the Commissioners of Northern Lighthouses (Northern Lighthouse Board) between 1837 and 1921.
The lightkeeper registers have two parts. The first has individual pages for each lightkeeper. The second consists of individual pages for each lighthouse, which lists all lightkeepers who served there. After signing in, choose the relevant selection to search by either individual lightkeeper, or by individual lighthouse.
I found 20 Macdonalds and 10 Mcdonalds. The search is case-insensitive.