Putting in my oar on the Mick/Mack controversy: My family have always pronounced Mc as Mick—although the vowel is elided so quickly that it's really a schwa. My understanding is that it is indeed an abbreviation of Mac, but this is the preferred spelling and pronunciation among the Scots who emigrated to Ireland beginning in the 16th century. And yes, I have always been taught to write it as a superscript with a macron under it, although I rarely see that in type.
An exception is in names where the actual surname following the prefix begins with a vowel, as in McIntyre: in these cases Mc is pronounced Mac, and also becomes the stressed syllable.
(Everyone I know with this type of name identifies as Scots-Irish; I am not sure about native Irish names, but I believe it's not so common: O'– meaning pretty much the same thing. But I have seen the name of the legendary hero Fionn Mac Cumhaill spelled Finn M'Cool in popular fiction, and I would assume here also the apostrophe would be pronounced as a schwa.)