There will be a new series of Y chromosome DNA tests available from the Family Tree labs which has the potential to settle many research points concerning the McCains. The new series of test will basically qualify degree of cousin your matches are. The procedure is different in that it will use both Y-DNA and mt DNA testing (as I understand it).
So, all the McCains from the Marsh Creek settlement, all the lines in Ireland that we have found, we can now, in theory, tell which particular line a McCain match belongs. Incredibly advanced molecular research really. This is done by measuring the autosomal DNA contributed by all ancestors.
I will post more on this as information becomes available.