Hi Carolyn,
Thanks for joining us here with your information!
As you say, there isn't a Belladuff townland to be found, it maybe that the place was misheard when adding to the headstone.
There is a townlands website that has lists of the townlands, here is the one for
County Cavan. There isn't a close match for Belladuff and the only Castlereagh references are for County Roscommon and County Down, both are close counties but neither border Cavan to be a bit of a border move.
Maybe, some of our members will have better luck with an idea of possible places.
How long have you been tracing your family tree?