Rosena,
I have not been able to contact a local volunteer. Sorry.
You haven’t said what year your ancestor died, but you do say that there are no interment details for the 6 O’Neill plots. That’s not unusual for Catholic burials. Most Catholic churches didn’t keep burial records. In this case there are burial records for the years 1818 - 1862 but there don’t appear to be any after that. I assume that is too early for your enquiry but if not, a copy is held in PRONI (the public record office) in Belfast. Free to view there.
If there are no interment records for the year you are interested in, there is no easy way round that. Only the family knew who was buried where, and as they died out the knowledge was lost with them.
Lack of burial records is very common. I once asked the supervisor of an old graveyard in Co Antrim how, if there were no burial records, they knew whether a particular grave was full or not. He told me they do a careful test dig, to see if there is any space left.
Undertakers sometimes have good records, for more recent years anyway. If you know which undertaker dealt with the burial, you could try contacting them.