Jim, Thanks for the reply - we should probably keep each other in mind. I'm a little rusty on family research and haven't done any for about 5 years but happened to stumble upon this site recently and your post caught my eye.
The parish priest provided the names of the children from the marriage of Archibald and Mary but was never able to find the marriage record - the dates I have for each sibling are Baptismal dates - he mentioned at the time that they may have beeen married in a neighbouring parish - I'm not sure where Co Armagh is in relation to Co Tyrone. The odd thing here is that with my earliest sibling being born in 1842 your marriage date of 1840 could fit in nicely.
Re the Griffith's valuation - does it show your Archibald as being in Co Armagh?
On a personal note I have a son Luke and my father's name was Jim.