All of this digging around for McKinley fore-bearers has left me with a meaningful gap in the line: in the latter half of the 17th century, ie post-1650 we have documented evidence of McKinley families in Donegal, Londonderry and Tyrone. We also have a McKinley in Sligo, but he didn't seem to have stayed put for long. Then if I work backwards I have my Sligo McKinleys clearly originating from eastern Mayo, with the earliest (or rather the oldest) there having been born less than a hundred years after the McKinleys listed in the various Hearth Money Rolls of the 17th century. Just to add to the problem, we are looking for records in one of the wettest, dampest and least developed of Ireland's counties: Mayo.
What I have recently found is that two McKinley's claimed for damages from the rebellion of 1798. The claims were lodged by Robert and John McGinely of Newtownwhite for the amount of twelve pounds and six shillings for Robert, and a smaller amount for John. Newtownwhite was on the route the 1000 French troops took to get from Kilalla to Ballina, so the farms would have been sacked. The amounts suggest that Robert had had more to lose; but the important fact is that the two names likely match John (Mayo Generation 2.0) and either his father or brother, who held a separate and more valuable holding. Hence Robert could be the original Mayo patriarch, or eldest son of the same.