Irish Family Names
Hereditary family-names became general in Ireland about the time of Brian Boru, viz. at the end of the tenth and the beginning of the eleventh century: and some authorities assert that they were adopted in obedience to an ordinance of that monarch. The manner of forming the names was very simple. Each person had one proper name of his own. In addition to this, all the members of a family, and of their descendants, took as a common surname the name of their father, with Mac (son) prefixed, or of their grandfather or some more remote ancestor, with Ua or O (grandson or descendant) prefixed. Thus the O'Neills are so called from their ancestor Niall Glunduff, king of Ireland (A.D. 916), and 'John O'Neill' means John the descendant of Niall.
END OF CHAPTER XV.