Where did you get the info regarding location and date of c1780-1820?
Hi Sancti
I must confess I have had much help on rootschat before but the Williamsons have always proved a problem. It is very confusing to explain but I will have an attempt.
The dates I quoted were "guesstimates" (if there is such a word). On my gtx2 grandfather's marriage certificate dated 1 July 1855 he is 35 years old, hence 1820 for his approximate birth date in the north of Ireland. His parents, stated before, would presumably have been born in the previous century, I imagine. In the 1851 census (Twynholm) there is a John Williamson, married to a Margaret Williamson, with two sons. I did wonder if this was him as on the 1855 marriage certificate to my gtx2 grandmother, Jane Marshall, he was a widower with 2 living male children! In Family Search there is a marriage of a John Williamson to a Margaret Halliday in December 1842 at Moira, Down, which would tie in with the first born son's birth date.
How can I obtain the marriage details of this John Williamson's marriage to Margaret Halliday at Moira but even then it may not show the groom's parents details. Where were the older John Williamson and Helen McColville married? As the marriage to my gtx2 grandmother was a Roman Catholic marriage I would imagine that previous births and marriages would have been recorded in some Catholic Church somewhere in Ireland.

Sorry about this complicated scenario but it's been puzzling me for years. I know that there were many records burned in Dublin in 1922 so I was just hoping that some original church documents may have been retained within the individual church buildings.
Do you think I should approach the Ireland History Society and pay for an official search?
Judy