Thank you very much for the quick response. I am working backwards from my ancestors John Connell and Sarah Toner, who had three daughters: Lucy, Mary (my ancestor) and Sarah. By 1872, the family was in Scotland. A poor law application indicated John was from Londonderry. His death certificate gave his parent's names as John Connell and Janet Kane. Sarah's death certificate listed her parents as Philip Toner and Jane Blair.
I recently located baptismal records for Lucy and Sarah. Lucy was baptized in Feeny on Oct. 23, 1848 (the sponsor was Mary Toland). Her sister Sarah was baptized in Feeny on Sept. 28, 1852 (the sponsor was Mary Anne Toner). I haven't looked at the parish records themselves, but the online index doesn't refer to Mary's baptism.
I was hoping the 1831 census would turn up something for the illusive Philip Toner or John Connell, the fathers of my ancestors, in Feeny or perhaps Dungiven, where there seemed to be more Toners. Maybe the death record was wrong, and Sarah's father was not Philip.
In any event, if you have any suggestion about where I might look next, I'd be very grateful.