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The plain truth is nobody knows for certain. It is usually explained as a variant of MacArtan/McCartan, which is an Irish name. McCartney originated in the South-West of Scotland, where Gaelic was spoken, but never written. Thus we have no early Gaelic forms of the name which could give clues as to its meaning.
The name seems more common in Ireland today than in Scotland. It was brought there in the 16th century by Scottish planters. Later some Irish with similar sounding names, MacCaugherty and Mulhartagh, adopted McCartney as their surname.