Hi dalybabe,
Your ancestors are indeed buried in Airth Castle graveyard!
The following details have been taken from the East Stirlingshire pre-1855 Monumental Inscriptions:
Entry No. 134
Flat Stone
1786
John Dewar and Christian Drummond.
There is also a description of the stone listed saying it is inscribed with three hunting horns? and some sort of animal.
Entry No. 133 is also for a Dewar . . .
1849
Charles Dewar died 19.11.1848 naged 56
Wife Ann Hutton died in Edinburgh 31.05.1873 aged 63.
Is it the Dewar or the Drummond line you are researching? If it's the Dewars, then I know someone who is also researching this line, she is also a member of Rootschat.
This graveyard was surveyed back in the late sixties, early seventies so I don't know whether or not the above stones will still be legible. I don't live that far from Airth so I'll try and put aside some time to go and have a look and take some photos while I'm there.
Anne.
P.S. From the pre-1855 book, there is a rough sketch of where some of the stones are situated so it should be relatively easy to find. Hopefully they'll be legible as I don't believe in removing moss etc.