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This is a very rare spelling. A few in lowland Scotland in the 19th century, and fewer still in County Mayo, Ireland, 19th century.
It has not been noticed by Edward MacLysaght (Surnames of Ireland), Patrick Woulfe (Irish Names and Surnames), or George Fraser Black (Surnames of Scotland); not noticed or listed as a distinct surname or as a variant spelling.
I wonder where you got the information that McKeay is a variant of McKeag? I think it more likely that it is version of Mackay or Mackee. Both of these names mean "son of Aodh", on old Gaelic name which non-Gaelic speakers have difficulty pronouncing. For a start the D is not pronounced, then the AO combination seems to be pronounced differently in different dialects. The result is many English language spellings: McKay, McKee, Magee, McCoy, MacHugh.
MacAodha, MacAoidh, and MagAoidh are Irish spellings; MacAoidh is the Scots Gaelic version.