In answer to one of my postings on the www.genforum.genealogy.com McCown Surname Bulletin Board, in 2002, one respondent mentioned the account of Bissett moving to Antrim from Scotland and adopting the surname McKeown. It seems that some Irish McCowns changed their names to McKeown. Over time, and possibly in the British effort to eradicate Gaelic names, some of the McKeowns changed their surnames to Johnston or Johnson. Many more chose Johnston than Johnson.
The reason that this comes as a revelation to me at this late date, is that I now have a 61/67 Y-DNA match with a Johnston(e). At first I assumed that this was a Scottish match, first because I thought the name was Scottish and second, because this match's earliest known ancestor was born in Dumfriesshire Scotland in 1805. Barry McCain pointed out that it was also an Irish surname and occurred frequently in Fermanagh Co., Ulster. Barry also pointed out that many Irish moved to Scotland during the Industrial Revolution for job opportunities. This in turn could explain how an Irish Johnston(e) happened to be born in Scotland in an area rife with Johnston(e)s of the Scottish border clan, in that it was in southern Scotland that the Industrial Revolution had most effect.
It sounds as though McKeown, Johnston(e) and Johnson surnames may all have to be considered in our genealogy. I was doing a Google search on the McKeown name and one of the results that came up was the aforementioned 2002 posting to Genforum.