Has anyone noticed the phenomenon of Mac names being changed to Ó, or vice-versa. An example would be Séamus Mac Grianna going to Séamus Ó Grianna. Womens names seem to be especially susceptible. I've seen more Ní Choitir's than the more correct Nic Coitir. Is the change legitimately accepted (obviously no one is literally "Mac" anymore, we're all more correctly Ó) or is it a case of people making mistakes or making the name easier to say?

Actually, "Ó Grianna" doesn't exist as a family name, it is Séamus Mac Grianna who chose "Séamus Ó Grianna" as his pen-name. His brother Seosamh kept the original name, Mac Grianna.
(I was told that by a university teacher of mine who was a specialist of these writers and of Rann na Feirste Irish)

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

There is no Nic/Ní distinction in traditional Munster Irish. According to Teach Yourself Irish, Ní is the female equivalent of both Mac and Ó, but outside of Munster the daughter of Mac __ would be Nic __.

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Táim ag foghlaim fós. Fáilte roimh gach aon cheartúchán.

Never heard that about Munster names. Does that apply only in Munster or can a Munster name be spelt like that regardless of where the woman is from. I knew about Ó Grianna being a pen name but it's interesting that he decided to change it anyway. Are there any other notable cases?