I have no direct answer to your query but The Index to the Prerogative Wills in Ireland lists James Fearne of the parish of St James, Westminster, Middlesex as a copy Will with the date 1720. There is also a Will listed for Margaret Fearne, parish of St James, Westminster, widow, copy Will 1720. It is interesting that these London Wills are listed in the Prerogative Wills of Ireland and means that they had to be proved or produced in Ireland for some reason. Going to the National Archives in London (www.nationalarchives.gov.uk) I find listed(and available to download online for a very small fee) the Will of James Fearne, corn chandler, proved in St James, Westminster 1696, and Margaret Fearne, widow of St James, Westminster proved in London in 1716. These are obviously the same people. I can’t be absolutely sure without seeing these Wills but suspect they might be the direct family of your Fearnes in Dublin, since both Wills had to be re-proved or presented in Dublin. It would be worth a look to see what these Wills say and what members of the family are listed etc., This does not directly answer your query, and could be a long shot, but might put you on to other clues regarding the possibly parentage of the Margaret and Sarah Fearne that you seek.