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

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

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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 Sep 14 '12 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 Nov 1 '12 at 13:21
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3 Answers

up vote 25 down vote accepted

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.

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Indeed, Mac is the Celtic (Irish Gaelic) equivalent of the son suffix in Germanic languages. There's nothing very special about it. –  Noldorin Nov 30 '10 at 16:07
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[pet peeve]There's no such language as "Celtic".[/peeve] –  Marthaª Nov 30 '10 at 16:11
    
There is nothing special about Mc or Mac, but there is (or was) something special about Ó –  Remou Nov 30 '10 at 16:23
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"Irish Gaelic" = Irish. Another peeve to pet. –  Remou Nov 30 '10 at 17:19
    
@Remou: indeed, most languages don't distinguish inherited surnames from literal ones. –  Marthaª Nov 30 '10 at 17:22
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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 –  Steve Melnikoff Nov 30 '10 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 Nov 30 '10 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 Nov 30 '10 at 15:01
    
In Ireland, yes. –  Remou Nov 30 '10 at 15:02
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@Claudiu: I think you missed the gender mismatch in @Remou's example. –  Marthaª Nov 30 '10 at 15:57
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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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