Irish patronymic surnames frequently feature the distinctive prefix O'. When these surnames were originally developed, they were formed by adding the Gaelic words O, Hy or Ui denote descendant of,  to the name of the original bearer's grandfather or to that of an earlier ancestor. For example, the surname O'Neil literally means descendant of Neil (Niall). While the O prefix is more commonly used today, ancient records include all of the above and have been used interchangeably.
In later times, these prefixes were also added to the occupation or nickname of the bearer's ancestor. By example, O'Riordan means descendant of the royal bard and O'Keeffe means descendant of the gentle man.
The Plantation of Ulster and the Cromwellian Invasion had a profound impact of the use of the prefix. The assimilation produced "degradation of the Gael and the inferiority complex it produced was the wholesale discarding of the distinctive prefixes O and Mac"
- ^ John O'Hart, Irish Pedigrees (1989 reprint of 1892) Genealogical Publishing Co. Inc. (pg 37)
- ^ Edward MacLysaght, Irish families (1972) Crown Publishers (pg 16)
- ^ Swyrich, Archive materials