The Recommencement of the use of ‘O’ in Irish Surnames, an example from the Girls School Roll, Carrigboy (Durrus) National School, West Cork, 1919.

For the ordinary Irish population for the 18th and 19th century names that habitually used the ‘O’ were written and spoken of without them.

Interestingly many Irish Protestant families of a Gaelic background high up the social scale used the ‘O’ such as ‘The O Donovan’ of Lissard.

The girls records of Carrigbui from 1866 are available and the ‘O’ is rarely used. this is an example of a girl, entry 791, Mary O’Brien, an orphan, from Friendly Cove.
https://plus.google.com/photos/100968344231272482288/albums/6146640094366678001

One of the peculiar aspects of the current usage is that some names which historically never had the O now have it attached.

Irish Surnames of Gaelic Origin