I hate thinking this is as far as I can get when a clan name like McDonald should be relatively easy.
Not so.
It is actually one of the most difficult, partly because it is such a common surname, partly because there are so many variants, and partly because the records from the more populous parts of Scotland are better, in general, than those from the clan territories in the Highlands and Islands.
According to 'Personal Names in Scotland', admittedly published in 1991, M(a)cDonald was the second commonest surname in Scotland (after Smith). I used the 'Home' search screen on SP to check the current numbers of the top ten surnames listed in that booklet. Sm*th is still the commonest, but M*c*d*n*l* with 1,330,882 listings is now third, having been overtaken by Wilson.
I note it's always spelled McDonald not MacDonald and I am unsure if that has significance either.
It has no significance in the vast majority of cases before the 20th century. Always search for M*donald, or even better M*d*n*l*, which will pick up the variants.