That is great that you have been able to get back that far. Mind you tombstones can & do often yield valuable information.
You have given me more food for thought about my search too. Can you tell me are you related to James Cummings & Anne Elizabeth Cummings?
Now dont worry about not finding information in Dunboe or Ballykelly, as sometimes clergymen may have mis-spelt names in their parish register. Also have you tried variations of the Cummings surname, Cummins, Commins, Cummings....
why I ask is on each of my Campbell ancestors birth & baptismal certs their Mothers maiden name was recorded differently each time as Cummins, Commins, Cumings, Cumins,