Thanks aghadowey. The date fits perfectly and thanks for searching. it does throw up a few considerations though. Frances did not have a sister called Margaret - but she did have a sister called Martha (who was called Mattie). So is Martha a variation of Margaret?
Frances birth cert clearly registers her as "Fanny" and if we dispose of the formal name, Frances, then her death cert fits well and carries the name she was called at birth (Fanny). It would be interesting to see if her actual Baptismal Cert had Fanny also!
It was an assumption among the family members at the time that she let home for Belfast and was never heard of again after that. As it was something that was passed down from one member of the family to another, then there is always the possibility that it was incorrect and not Belfast she went to at all, but perhaps Kildare or Dublin.
So many young Irish girls fell pregnant and had to go elsewhere to have their baby as the Catholic faith was very strong in Ireland in bygone days and several institutions took these young girls in, where they had their child, which was later adopted and the mothers often were prisoners after this and not able to leave the religious institutions. It was regarded as a shame on the family if the Parish Priest heard of such a case and would think nothing of announcing this to the entire parish and to avoid such, these young girls just disappeared at the time.
Perhaps Fanny met with the same fate. I wonder if there was such an institution in Leixlip, and perhaps she died during childbirth. The death cert (if hers) does not elaborate more than just Leixlip as the place of death.