I’m sure you are familiar with the naming convention of many Celtic peoples where a son is named after his father or at least a notable fore-bearer. Among the Irish you have Mc and Mac and O s which are really Ui s. The Welsh use Ap and Map the same way. In short O’Neill means grandson of Neill whereas MacNeill means son of Neill.
How else to keep separate all the Anguses and Rhaurys and Eochaids? Ah, well, before the application of the now traditional surnames, notable personages received Bye Names (at least we have theirs recorded, likely everybody had them). After all, in the list of High Kings of Ireland alone there are plenty of Eochaids and Aedhs and Nialls. Fortunately there are nickname like descriptive names that when added to their given names help to keep track of just which one you are talking about.
Interestingly many people who’s surname is O’Neill or MacNeill might think that their name comes from the famous Niall of the Nine Hostages. They may share genetics with Noigiallach but the name comes from a later Niall, Niall Glundubh, that is Black Knee.
Much more mysterious to me than how Niall Nine Hostages got his name is what in the world happened to stick Niall Glundubh with the bye name Black Knee. Does anyone out there know the origin of Black Knee?
All in all, from Nuada Airgetlam (silver hand) to Conn Cetchathach (of the Hundred Battles) to Eochaid Mugmedon (Slave Lord) to Elim Oillfinshneachta (Snow that tasted of wine) to Finnachta Fleadhach (the festive) Bye Names are just good clean fun.
Write me down as a yeah vote on renewing this tradition.