Dayle,
The 1821 census for Armagh was mostly destroyed in the 1921 fire in the Public Record Office in Dublin. Only a few fragments survive, for the parishes of Kilmore and Forkhill. So there were many other McCanns across Co. Armagh in 1821. Just that those records don't exist. Looking at Griffiths Valuation for Armagh in 1864 there were 465 McCann households, spread across most of the parishes in the county. So you need to be cautious when concluding the Forkhill families might be your family.
Forkhill RC parish has no baptism records earlier than 1845 so unfortunately it won’t be possible to check whether the family were there before that date, at least from those records.
There are papers in PRONI relating to Henry Alexander. However I don’t see any tenants records, so I’d suspect those have not passed into public ownership (if they still exist). My general experience of estate records is that all the estate recorded was the name and townland of the person paying the rent. That’s all they were interested in for obvious commercial reasons. They had no interest in the tenant’s family or dependents and so even if the Alexander estate records survive, they are unlikely to provide any information about tenant’s dependents. (There are exceptions with 3 lives leases, usually granted in the 1700s, where sometimes several members of a family were named but that was not always the case).
The other thing we need to know is what your McCann’s occupation was in Armagh. If they were farmers, then they were likely to hold leases from a landlord like Henry Alexander. But if they were agricultural labourers or weavers, then they would have been living on somebody else’s farm, and renting from that farmer, not from the main landowner. So that can affect the type of records you are looking for. Do you know Hugh McCann’s occupation in Ireland?
Possibly DNA testing may be a way of matching with others who have additional information about where the family originate. Family Tree DNA reportedly has more people with Ulster roots than any other company. That obviously increases the chances of finding a match. You might want to try them or, if you have already tested, you can transfer your results to them for no fee.
The North of Ireland Family History Society are running an Ulster DNA project and can offer FTDNA testing kits at a reduced price. http://www.nifhs.org (Go to DNA project on the website).
