|Meaning||Derived from the elements ua and Néill, meaning "grandson of Niall."|
|Region of origin||Ireland|
|Related names||O'Neil, O'Neall, O'Neal, Ó Neill, Ó Néill, Ua Néill, Uí Néill and Neill O'Neal, Oneal|
- For the histories of the prominent families, please see the article O'Neill dynasty.
The surname O'Neill is an Anglicization of the original Gaelic Ua Néill, composed of the elements ua, meaning "grandson" or "descendant," and of the Gaelic name Niall. The meaning of the Niall is disputed, but has been suggested as "cloud", "passionate" or "champion". The progenitor of the family is said to be Niall Glúndub of the Cenél nEógain, however it would have been his great-grandsons, who lived in the tenth century, that would have been the first to use the surname.
It is due to the Anglicization of the original Gaelic that the several spelling variations have emerged, during the transcribing of the name into English. As well, all variations upon the O'Neill spelling are incorrect. This is mainly due to the lack of literacy and ability to spell, (common of the times) and people wishing to associate themselves with the O'Neill royalty. Irish and Scottish variants also exist and include MacNeal, MacNiel and MacNeill, which arose when the ua element in the name was replaced with mac, meaning "son of." Ó has replaced Ua since the end of a standard Gaelic and its gradual evolution into Scottish, Manx and Irish. O'Neill is also occasionally found used as a given name.
Coats of arms
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It is a mistake to state that the Irish coat of arms system follows a feudal system wherein a coat of arms is property passed through direct lineage. This means that the right to use the arms is not restricted to a given individual, as in the English feudal system, but is open to all within the extended "sept" or "clan" of the Gaelic culture.
The coat of arms of the Uí Néill (plural of Ó Néill) of Ulster were white with a red left hand cut off below the wrist, palm facing down with the fingers spread. Today, it is more common to see the right hand, palm side up and with the fingers touching rather than the left, as the coat of arms was changed under British rule. It has also become a symbol of Ireland, Ulster, Tyrone, and other places associated with the ruling family of Uí Néills.
The symbol is frequently used by Protestant inhabitants since the 1920s in Northern Ireland. As other related family branches and clans loyal to the O'Neills were often granted or assumed a heraldic achievement, this red hand has been incorporated into the new coat of arms to the point of being a cliché.
The red hand is explained by several slightly differing legends, most of which tend to have a common theme beginning with a promise of land to the first man that is able to sail or swim across the sea and touch the shores of Ireland. Many contenders arrive, including a man named O'Neill, who begins to fall behind the other. Using his cunning, O'Neill cuts off his left hand and throws it onto the beach before the other challengers are able to reach shore, thus technically becoming the first of them to touch land and wins all of Ireland as his prize. However, the legends seem to originate in the seventeenth century, several many centuries after the red hand was already used by the O'Neill families. 
- A Dictionary of First Names. Oxford University Press. ISBN 0-19-280050-7.
- "FAQs: heraldry". College of Arms. Retrieved 2016-12-26.
- "Heraldry in Ireland". Heraldry.ws. Retrieved 2016-12-26.
- "Pat Brennan Heraldry". Leitrim-roscommon.com. Retrieved 2016-12-26.
- "O'Neill coat of arms". Araltas.com. Retrieved 2016-12-26.