I couldn't find a Martha Isaac among the baptisms. There was a Margaret, baptised 21 May 1818 in Llansamlet parish, which is in the Swansea area. I suppose she could (just) have been a Margaret who was known as Martha. But this one's mother was Mary, rather than Margaret ( father was Thomas, a labourer, and they lived at Tynyfron).
Not too many of the entries have the mother's name included, and the only Margaret Isaac I found was married to a John and had three children around that period, but no Martha - and they were in Eglwysilan parish.
Looks as though we've drawn a blank there, I'm afraid, but when you get Martha's marriage cert I can try again for her parents' wedding.
Gareth