You can try putting the name Duncan in the name search on rootsweb and anybody interested in the surname and has been on rootsweb looking for the name Duncan should be there.
The earliest records used as a census substitute are:www.genealogy.nationalarchives.ie
The Tithe Applotments 1820-1839
There were 120 Duncan's occupying land in those years.
There is a Charles Duncan, Parish of Drumreilly,
Townland Shenaghmeeleen, Co. Leitrim 1834 but there are no Stanton's in Leitrim. So this would probably not be your
Charles Duncan. Generally people married those in their own parish's or at least fairly close by.
I assume you don't know where in Ireland so this will make it a difficult search as there is so much genealogy history missing. Parish records do exist that early but not all.
Read about the Four Courts Fire 1922.
So you need to find at least a county to start to find a common county. One method is the next census substitute
Griffith Valuation, I would suggest you use:www.failteromhat.com
first as it just lists name, parish, townland and counties. If you find a county in common just
search those. You should be able to do this.www.askaboutireland.ie
once you have a county you can use this resource for Griffiths with more detail, landlord, property, neighbours.
Most of Ireland's counties (-4) do belong to an Irish History Foundation with bmd parish records. I can check with just a name, Charles was fairly unusual at that point in time unlike John, James, Patrick....
I would try looking for the children since you have both parents name, you put those details in and you should be all the children that were born in Ireland. This would give you a county, place/parish. This is a pay website as most of
Irish genealogy comes with a fee. As they charge a fee I cannot give you the website due to rootsweb policies. You can contact me at firstname.lastname@example.org
Note that currently the County Down site is undergoing updating. So you need to wait. I was unable to see how far their records went back for either the Church of Ireland or Methodist.
I found only 1 Parish in Down that that both Duncan's and
Stanton's. The Griffith Valuation took place in Co. Down in 1864 which is a lot further off 1840 that I had hoped.
Parish Newry seems to be the common Parish. I will look tomorrow it is getting late.
There are websites that will do a little pre-work for you
regarding the counties at least matching up the surnames where they occur the most. This is also a pay website and they even charge for Tithe's and Griffith's which are
A Parish maps website shows that County Down has both surnames. So this is where your first reply came about
another bmd search