Morning
Interesting family...
If I am right, you can follow them back beyond 1700 living around the small-holdings at Little Kilvelgy and Kilvelgy Wells, both of which now mostly been subsumed by the modern development around Kilgetty. Poll books, wills and estate leases are the source for this conclusion.
There are two marriages for an Isaac Welch in the local marriage registers around this time:
20/11/1802 married at St Issells church (bachelor, OTP) to Martha Williams (Sp, OTP) witnesses: John Prickett (parish clerk) and John Griffiths
Three baptisms to this marriage
4/3/1815 married at Begelly (widower, St Issells) to MAry Waters (sp, OTP) witnesses: James Waters and William Eynon
Three children baptised to this second marriage: 2 at Begelly, 1 at St Issells. The last one died in a pit accident in 1843.
Also details on 2, maybe 3, illegitimate children.
I saw your other post about Mary Walters. Let me throw a spanner in the works by suggesting that her surname could be Waters. There are several occurences in the Begelly/St Issells registers of confusion over these two surnames. You'll see that "James Waters" is a witness to her marriage. (that's how I transcribed it anyway). There's a James Waters living in the area at the tme, aged abt 30 at Mary's wedding. I can't see a James Walters though.
If I extend my conjecture a bit, this could make Mary Waters the sibling of James and therefore daughter of William and Mary Waters of Begelly (around the Thomas Chapel area). There's no baptism for Mary in the Begelly registers but her approx birth date of abt 1778 fits in nicely with baptisms and assumed birth dates for other children of Wm and Mary.
There's no obvious burial for Isaac Welch in the Begelly/St Issells burial registers.
Jon