Hi Garth - No I have no documentation thus far - just joing up the dots on IGI etc.
What does encourage me though is that they appear to be the only Handerson family in the area, plus that the area concerned is very small and not far from Loughgall where Isaacs Grandson Thomas (my 2xGreat Grandfather) married Elizabeth Welsh in Loughgall Parish Church on 06/10/1853. I do have that documentation as well as Thomas' death cert which shows his parents as Joseph Henderson b 1804 in Syerla and Nancy Allen.
Isaac Married Jane Askin in 1798, the same year that James Henderson was born, followed by Isaac jnr in 1801, Ann in 1820 and Joseph in 1804.
I have in my notes that the link between Isaac and his father, Thomas b abt 1746 and married to Mary Unknown, I got from One Great Family.com
Trivia - It was Thomas and Elizabeth who immigrated to New Zealand where I was born. I have a photo of their headstone at the Churchyard where my brother was married in 1968.
I hope you find this helpful - Evan