Both Irish and Scottish use “Mc” and “Mac” variants actually. The Gaelic word “mac” means “son”; Viking and Saxon influence is why you have both “McDonald” and “Donaldson” in use in the British Isles.
Re: Mac vs. Mc, it is not nearly absolute, but much more commonly divided as I suggest.
And the McDonalds who founded the restaurant that the chain grew from, again, were of Irish, as opposed to Scottish, descent.
It is not common, but I have seen the name "Fitzdonald" which shows Norman French influence.