Hi there I live in the townland of Ardkill, Ardmore. It is a rural countryside location, located about 6miles from the city of Derry. My family have only been living/farming in Ardkill since the 1970's.
There is an old derelict house/mansion on the Ardkill Road called Ardkill House. It was owned by wealthly landowners back in the 1800/1900's, I think their surname was Stevenson. Maybe your relative worked there?
You mentioned a surname, Sherrard. I know that in Ardmore there is still a family with this surname living in the area.
If you go to multimap.com, select Northern Ireland then input BT47 3RH into the postcode field you can view a map of Ardkill. Let me know if you want me to find out anything more specfic...