Just to be pedantic (why not join in) the O in front of Irish surnames represents grandson of, or descendent of rather than Son of, which is Mac both in Irelad as in Scotland. The Gaelic version of most surnames still retain the Ó or the Mac whereas it is dropped in some versions of the Anglicsed names. Thus I know a number of people who's names are Ó Ceallaigh in Gaelic, some call themselves Kelly in English, others O'Kelly.

As far as I can remember from my History lessons, surnames were imposed on us by the British and did not exist in ancient Gaelic. The practise of using surnames became widespread only after the practice of Primo genitur was introduced in the reign of Henry VIIIth as the rule for inheritence of property. In Gaelic speaking areas many people are still known locally by their given name followed byh that of a parent and Grandparent e.g Mairtin Seamus Seosamh.