As someone with a Mc surname and Irish ancestry, I can vouch that pronouncing McDonalds ‘Muck’ is correct. Mick is ….acceptable ish (but very clunky sounding to British/Irish ears) but definitely not Mack as it’s McDonalds not MacDonalds.
Mac is (more often) the Scots way. Both mean ‘son of’ Mc is more likely Irish, Mac is more likely Scottish. So someone with the surname MacGuire would be ‘Mack Guire’ and someone with the surname McLean would be pronounced ‘Muck-lean’ or ‘Muh-Clean’ (just like the F1 team McLaren is pronounced ‘Muh-Claren’ not ‘Mick-Laren’ or ‘Mack-Laren’
and that’s the end of the lesson today folks
I’m a bit of an etymology geek if you hadn’t worked that out