Elizabeth Mullan, born 1 Nov 1846 in Brockagh, father recorded as James Mullan, mother is Eliza McGuigan. Info from the Birth Record book held at St Mary's Catholic Church, Ballerin, Garvagh, Coleraine, Londonderry. Could be Nancy's sister. There are two women/mothers also listed in the book with the maiden name Gribbon, Anne and Bridget, they are both from Drumbane. I believe Slaght and Brockagh townlands are pretty close to each other, Drumbane as well. I think they would have to be since they are listed in a church birth record book. Looking at the records, Mullan and McNichol are more common surnames, it doesn't seem like Gribbon and McGuigan are as common. The names you list in your post are definitely from this area there are also a bunch of McNichols.