A very sincere thank you to Elwyn for his information about the Griffiths Revaluations and finding out what happened with my Johnston ancestors in Derrykeighan, solving the mystery of centuries which have puzzled us for so long. Not only that, he provided background information which was really interesting, and I have his permission to post to the list, to help others who don't live there and know it.
I found that the columns in Griffiths Valuation need explaining (well, to me).
'Townlands and occupiers' were the people who rented the property,
'Immediate Leasors' were the people who either owned the property or were leasing it on behalf of someone higher up the chain, like the Earl of Antrim.
I'll post some of Elwyn's information separately.