Irish patronymic surnames frequently feature the distinctive prefix O'. When these surnames were originally developed, they were formed by adding the Gaelic words O or Ui which mean 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 Niall. In later times, these prefixes were also added to the occupation or nickname of the bearer's ancestor. For example, O'Riordan means descendant of the royal bard and O'Keeffe means descendant of the gentle man.