As a follow on to my original question, I have now found this registration of death for 1784 in Kingarth, Bute. I transcribed it as follows:
Dec 16th Janet MacConachy spouse to James Macfee in Buttcurry died of the Histericks (?) upon the sixteenth and was buried upon the seventeenth instant aged about 40 years and paid the Kings dues.
I am confident the registration relates to the headstone because the names, dates and quite specific area, Buttcurry, are the same apart from the spouse’s surname. I think MacConachy means “ son of Duncan “ , but as anyone come across this before?