There is clearly a prefix in names like McDonald, McChrystal, O’Brian, O’Neal.

What does this Mc- and O- prefix signify? It looks like Donald, Chrystal, Brian, Neal are perfectly fine names on their own, so why is there a prefix before it?

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    You forgot to mention another prefix for names, that is "Fitz" as in Fitzgerald. It also means "son of" and it clearly shows noble French Norman ancestry.
    – Paola
    Commented Sep 14, 2012 at 19:54
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    @Paola: "fitz" can be derived from Norman French, true, but that does not mean someone with a surname in Fitz- necessarily has Norman, French, or noble ancestry.
    – Marthaª
    Commented Nov 1, 2012 at 13:21
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    @Marthaª Fitz was used for bastard-names, so FitzRoy was a natural son of the king. There are also various sorts of FitzWhatevers, like FitzWilliam or FitzCharles or FitzClarence — all originally “illegitimate” children, but sometimes not forever staying that way.
    – tchrist
    Commented Dec 7, 2014 at 23:56

3 Answers 3


Mc is an abbreviation of Gaelic Mac, "son".

The standard way to form a name using a simple patronymic byname for men is:

    <single given name> mac <father's given name (in genitive case & sometimes lenited)>

which means

    <given name> son <of father's given name>

For example, Donnchadh who is the son of Fearchar mac Domhnaill would be:

    Donnchadh mac Fearchair

which means

    Donnchadh son of Fearchar

O' is the Anglicized way to write Ó "male descendant of".

The standard way to form a name using an Irish clan affiliation byname for men is:

    <single given name> Ó <eponymous clan ancestor's name (in genitive case)>

which means

  <given name> male descendant <of eponymous clan ancestor>

For example, Donnchadh who is the son of Fearchar Ó Conchobhair would be:

    Donnchadh Ó Conchobhair

which means

    Donnchadh male descendant of Conchobhar

Two common misconceptions are (1) that Mac means "son of" — it actually means just "son", and the "of" comes from putting the father's name into the possessive case; and (2) that Mc is Irish while Mac is Scottish (or vice versa) — actually, Mc and Mac are two ways to write the same thing, and both occur in names from both countries. (What is true is that O' is almost exclusively Irish; despite the romantic notions we have of Scottish clans, they didn't use their clan affiliation in their names.)

Edit: as for why the prefix is used even though the prefix-less names look perfectly fine on their own, this is basically Gaelic grammar and thus out of scope for this site. Suffice it to say, some languages are fine with unmarked patronymics — names that identify the bearer's father using the unmodified given name — but Gaelic is not one of them.

  • Indeed, Mac is the Celtic (Irish Gaelic) equivalent of the son suffix in Germanic languages. There's nothing very special about it.
    – Noldorin
    Commented Nov 30, 2010 at 16:07
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    [pet peeve]There's no such language as "Celtic".[/peeve]
    – Marthaª
    Commented Nov 30, 2010 at 16:11
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    "Irish Gaelic" = Irish. Another peeve to pet.
    – Remou
    Commented Nov 30, 2010 at 17:19
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    At least in Irish (not sure about Scottish), ó specifically means ‘grandson’, though in surnames it is better to see it as just a vague ancestral relation (basically anything further removed than ‘son’). And using ‘Gaelic’ on its own to refer to both Irish and Scottish—as well as Manx, by definition—is generally frowned upon in the field. ‘Goidelic’ would be a better term (Brythonic has the same patronymic, of course, but it is (f)ab in Welsh and other forms I don't know off the top of my head in Cornish and Breton). Commented Feb 12, 2014 at 9:10
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    @JanusBahsJacquet: note that putting the father's name in the genitive is a type of marking, so this is still not an example of an unmarked patronymic. It's Fred Johns, not Fred John.
    – Marthaª
    Commented Mar 29, 2015 at 2:40

Mac, is the Gaelic for "son", and O' means "grandson of". It is found mainly in names from family of Irish origin.

See Wikipedia for more information.

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    The prefix "Mac" is closely related to Scottish Gaelic as well; see en.wikipedia.org/wiki/Scottish_Gaelic_personal_naming_system Commented Nov 30, 2010 at 12:32
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    Not O', O (Ó Domnhaill) the apostrophe was due to a confusion with the English O' (of). And do not forget Nic, Ní and Uí
    – Remou
    Commented Nov 30, 2010 at 14:57
  • @Remou Do you happen to know if Nic, , and are seen in modern naming convention, as it is the case with Mac and Ó_/_O'?
    – Eldroß
    Commented Nov 30, 2010 at 15:01
  • In Ireland, yes.
    – Remou
    Commented Nov 30, 2010 at 15:02
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    @Claudiu: I think you missed the gender mismatch in @Remou's example.
    – Marthaª
    Commented Nov 30, 2010 at 15:57

I found the following quote which could be helpful. It is from David Booth's (1766-1846) book: An Analytical Dictionary of the English Language.

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"Words in ITE very generally denote one of a tribe or nation, and as such may be taken substantively, and have the plural. The Old Testament is full of such denominations, such as the Hittites, the Midianites, &c. Like the ides of Homer, they bore the name of their ancestor. The Israelites were the children of Israel, as the Danaides were of Danaus ; in the same manner as the MAC (son) of the Irish, refers to the father of the tribe, to whose name the syllable is prefixed. Such PATRONYMICS (father-names), as they are called, exist among all nations."

Page xcvii (or pdf page 113) Source: https://archive.org/details/analyticaldictio00bootuoft

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