If one reads the transliteration of the Highlanders, one would today pronounce Mac Domnaill, as it looks, without having the ability of a dictionary coding, it would look like the universal NATO: MAAK dawm-nay-ell. If however one has been to the deep of Scotland, where American English has not corrupted the local tongue. Gaelic today is difficult to understand. And if one has been to a Robert Burns Celebration, the reading of the Haggis. gives one an idea, if Robert Burns was so close, and the first Kings of Scotland 1,100 years ago - try to imagine the difficulty of today's linguists. Old French is almost impossible, even for the professionals. So bear with me, as I attempt to speed up the clock and demonstrate a surname transformation.
Mac Domnaill would be heard as M'c Dooh Nall. Over many years, as in Spanish, the letter, as Americans look at it 'll' is a 'y' sound. Over time it would be very easy for a native speaker of Gaelic to start to pull the tongue back to a glottal stop, and that would with the proceeding 'lau' - the soft "L" as in lullaby rather than the hard "L" as in Look, the 'L' and the 'D' would be the resulting sounding diphthong. Hence, McDonald, or MacDonald would be the final product. With the preference being the Mc rather the Mac. One being a soft 'm' and soft 'uh' and a hard 'k.' In the other case, the correct pronouncement, would be a soft 'm' followed by a hard 'ah,' as in tack, which hurts when one sits on. The final sound would be again, as in the first, a hard, 'K.' It is only a natural evolution of sounds and not spelling, as we would do.