A half-sister?

The early Baptism Registers of St. Anne's Church of Ireland, Shankill, Belfast, have two baptisms which look as if they might be related for the surname of the first is McIlhaggy and of the second is McIlhaggey. What is more the name of the mother in both is Sarah. However, the children concerned cannot of course be full siblings for Margaret baptised on 4th March 1790 had Nathan as her father and William baptised on 25 February 1792 had James as father. I have mentioned these baptisms on a couple of occasions in this blog, and puzzled about them, not knowing which family they belong to. There are however a couple of things which occur to me now which might throw a little light.

My bafflement is not least because these folk were in the centre of Belfast. Where did they move from, and did they move away again? I have no other reference to the clan name there. The most likely thing seems to be that they moved from Islandmagee. However the parents must have been born as early as, if not earlier than, 1770, and I have no Nathan who might have moved about that time. There is James 'Junior' on Islandmagee, but I don't believe he moved away from the farm there. The next thing to note is that after 1790 there are no further references to a Nathan fathering other children. He disappears from the baptism scene. I have to suspect that he died at a young age, in which case Sarah would have been left with the child Margaret.

Now within a short time frame there are several references to a James with a wife Sarah, with other children who were baptised not long after Margaret, all at St. Anne's, Shankill. First there was William baptised 25th February 1792. Then, after a gap of six years, another William baptised on 29 June 1798. We have to assume that the first William died, maybe soon after birth, which could have made them avoid having other children for a few years. Then five years later there was Martha baptised 6 July 1803 and finally Ann baptised the following year on 6th September. In the Registers all have William and Sarah as their parents, though the surname gets a variety of spellings. As we have learned, that was to be expected at that period. William in 1792 was McIlhaggey; William in 1798 was McIlhagar (though Sarah was McIlhaggar); Martha was McIlhaga and Ann was McIlhagart.

I think these records do raise the question of whether by some misfortune Nathan died soon after the birth of Margaret and whether his wife remarried to James, very probably the brother of Nathan. This would of course make Margaret, assuming she lived, a half-sister to William and to the other siblings.
Now I have one further observation. I have recently learned the death date of my great-great grandfather William in 1882 at the age of 87, giving him a birth year of 1795. Given that many people were a little uncertain of their age, and other people reporting their death even more uncertain, and given that 1795 and 1798 are only three years apart, I have to raise the possibility that my William was in fact the William who was baptised in Shankill in 1798. I know of no other reference to a William with whom I can identify him, and I have on another occasion said that I suspect that his father was James, not least because in the Tithe Applotment Books for Ballycloghan and Kenbilly my William and a James McIlhagga rented neighbouring plots.