Right- think you've gotten yourself into a bit of a muddle and have been given incorrect or confusing information when you tried to get more details.
Firstly there are different types of records-
1) gravestone inscriptions (not always accurate as can often be erected years after an event, may not include all burials in that particular plot)
2) burial records (cemetery records or church burial register/records). What you probably don't know is that most Presbyterian churches that have burying ground did not keep records of burials and also that Presbyterians were at one time entitled to burial in the Church of Ireland graveyards. This was because everyone had to pay tithes to the Established (Church of Ireland) church.
I found this, which is the source you've now given-
http://kabristan.org.uk/kabristan-indexes/ireland-indexes/53-ireland-county-donegal-graveyards-part-i-vii/750-county-donegal-graveyards-bAt the very top is says:
Copies of full memorial inscriptions are available for £4.00 including postage. To order please use our order form. Please email * if you have any questions or require more information.This means that the details in this list are from a gravestone NOT burial records.
If you go down to the Burnet listings it says
Greenbank Church County Donegal Ireland which doesn't actually tell you the NAME of the church.
All this tells you is that there is a headstone with these names (or headstones- no way to tell is ALL the names appear on one stone together).
Assuming, and only as an assumption for illustative purposes, that all the names are on one stone it could read something like this- Andrew Burnett died 1870 his wife Kathleen died 1871 their daughter Martha J. died 1888 and her son Robert died 1888. Therefore Martha J. has been indexed under Burnett, as had her son, but she could have been married.
Without seeming the actual wording there is no way to know what the names, relationships, etc. are.I then emailed * and they told me Martha Burnett nee Foster was not my relative in that particular (I am assuming it was a grave) but I don't know.Your relative would have been Martha Foster nee Burnett.
If you give me a few minutes I'm going to see if I can find Martha's death registration for you.