The meaning behind the name McCairn begins when the name originally appeared in Gaelic as O Ciarain or Mac Ciarain. These names are derived from the word ciar, which means black or dark brown.

Numerous spelling variations of the surname McCairn are preserved in old documents. The various spellings of the name that were found include Kieran, O'Kieran, Keiran, Keighran, O'Keiran, and others.

This surname is first found in Mayo County, Ireland.