The families of McMahon and O'Mahoney are related only by the common derivation of their names from the Irish name Mahon (in Gaelic Mathuna), which was a personal name, meaning a bear. The O'Mahoney or Mahony family are derived from Mathuna, a grandson of Brian Boru. The family were the Eoganacht, the regal dynasty of Munster and were one of the most prominent of West Munster.
The O'Neill family traces its history back to 360 A.D. to the legendary warrior king of Ireland, Niall Noigiallach (Niall of the Nine Hostages), who is said to have been responsible for bringing St.Patrick to Ireland.