Two more queries

Still working through the Ireland, Births and Baptisms, 1620-1911, I have two more queries. One is raised by the birth of James Clarke on 27th September 1879 in Lislaban (Lisburn?) to Joseph Clarke and Nancy McIlhagga. Oddly the record adds that Nancy is the daughter of Jane McIlhagga. Why does it give a grandmother's name as well as the mother's? There is a Nancy, daughter of James and Jane McIlhagga (nee Maitland) of Ballyportery, Co. Antrim. I have had a tentative date of 1870 for the birth of this Nancy but this cannot be so if I have the right family. If James was Joseph and Nancy's first child we can assume they were married 1878/9 which would give Nancy a birth year of say 1860 at the latest. This in turn would have made Nancy the eldest child of James and Jane and probably born before their marriage. We have a birth date of 25 May 1840 for James and in all probability Jane was about the same age. Perhaps this is the reason for the note about Jane as the grandmother at the birth of James Clarke.

Another problem. I reported recently on the birth to John Fullerton and Mary McIlhaggar of Mary Ann in 1872 at Galgorm, Ballymena. I now have the birth of John to the same couple on 28th August 1879. The problem is that at present I have John married to Margaret daughter of James McIlhagga and Mary Ann Gard(i)ner, with a marriage date of 8th January 1872 at Connor Church of Ireland. Could Margaret have died and John have married her younger sister Mary who was born 8th September 1851?