Hi Ashe,
I too have John Morgan m. Mary Glancy/Clancy in the tree that I am working on.
I have been able to find a lot of information on the children of John and Mary - especially through Newspaper notices (Belfast), but nothing prior to their marriage in 1806 (Shercock, Co. Cavan).
Through the UK, British Army and Navy Birth, Marriage and Death Records, 1730-1960, I (and you also) may have seen the "Statement of Service of Quartermaster Mr John Morgan of the 52nd Regiment - born Bailieborough Co. Cavan Ireland DOB 24th June 1785. Age on his first Entrance into the Army, 22 years - 101 days."
Happy to share any further information with you - and perhaps you too have something to offer.
Regards
meg