I have occasionally seen the "Mc" part of such names, with the little "c" raised and underlined to emphasise its distinct meaning, which with deterioration of the generally exposed stone itself, could start to look like what your cousin may have seen.
From what I have read, the stone mason may not have been literate in the sense we know today, so could be "copying" a scrawled note, which he could not read in today's sense of that.
Imagine yourself trying to copy something in an "alphabet" or writing system unknown to you, such as Cyrillic, Pharsee, pictograms etc.
It may also be the way he heard the name being said, because of accents, brogues etc.
All sorts of things to go astray.
Other possibilities are misunderstandings due to intermingling the English alphabet, 26 letters today but 29 several centuries back, with the Scots Gaelic alphabet of 18 characters, if I remember correctly.
Take the Scottish resort known as "Aviemore".
In Scots Gaelic, it is "An Aghaidh Mhòr".
Or the Irish name "Siobhan" generally pronounced "Shi-von".
Regarding "Gael accent", that varies considerably, being generally softer up to the north-west and out to the Hebrides.
Something like that happens down in Cymri, the Principality of Wales, where my lady-wife comes from.
Southern Welsh sounds softer to me than Northern Welsh.