Mc (or Mac) is often used as a prefix in Gaelic-derived names.
In one class containing most such names, prefixing Mc does not affect the position of the accent somewhere on the base name. Thus Mc is unstressed. For example:
- McDonald /mɛkˈdɑnl̩d/
- MacArthur /mɛkˈarθr̩/
- McCoy /mɛk̚ˈkoj/
- McGill /mɛˈɡɪl/
- McMahon /mɛkˈmæn/
- McCarthy
- McLeod
- McDonnell
- McCormack
- McEwan
- McAllister
- McOrmond
McNuggets
But sometimes Mc takes stress:
- McIntosh /ˈmækɪntɑʃ/
- McIntyre /ˈmækɪntajr̩/
- McAfee /ˈmækəfi/
- McAvoy /ˈmækəvoj/
All of these I could think of contain a vowel-initial base name, but that's not a sufficient condition.
How can you determine which class a particular name belongs to, and what is the etymological reason for the distinction?