McClenaghanReferences to Origins and Meanings
First, some background information about ethnicity and names
The Gaels of Scotland are descendants of Gaelic settlers from Ireland. There is basically no language difference between Irish Gaelic and Scottish Gaelic. Scottish Gaelic was (or is) spoken by Scottish Highlanders. The lowlanders spoke another language, Lowland, a dialect of English. Irish Gaelic is now simply known as Irish.
Gael comes from an Old Irish word meaning Celt. Scot comes from an Old Irish word meaning Irishman. Although the"C" in Celt should be properly pronounced with the "K" sound, in America it has become corrupted.
There are words commonly associated with Ireland and Scotland, but "wee", "lassie and lad", "dale" and "dingle" are Middle English. "Glen" is both Middle English and Scottish, while "bonny / bonnie" is Scottish.
"Mac" is the equivalent of "son of" or "son" in English (e.g. MacWilliam would be Williamson). It also appears abridged as "Mc" or "M' ". All of these appear in both Scotland and Ireland. Regardless of spelling, it is properly pronounced "Mac", not muck or mick.
From More Irish Families (Edward MacLysaght, Irish Academic Press, 1982), we have McClenaghan as the anglicized form of the Gaelic name Mac Leannacháin
Breaking this down, we have "Mac" meaning "son of" and "Leannacháin", meaning little Leannach. Áin is the diminutive form in Gaelic, equivalent to "little" in English or adding "y" at the end of a name (e.g. Bob becomes Bobby). Leannach (or leanach) means "possessing mantles", a mantle being a loose, sleeveless coat.
In New Dictionary of American Family Names (Elsdon C. Smith, Harper & Row, 1973), the meaning given is: The son of the servant* of Onchu**; the son of little Leannach.
See also The Surnames of Scotland by George F. Black (1946). Here the name is listed under MacClannachan, with variations shown of MacClenaghan and M'Clenaghan. In this text, the meaning is given as Mac Gille Onchon, son of the gillie* of Onchu**.