I searched the subscription site Roots Ireland and there is no Thomas Denning baptismal record around 1792 in Co. Longford. However, I did locate a 1794 marriage record at the Ardagh and Moydow RC church. Husband was Thomas Denning and wife Margaret Corcoran Date Oct. 13 1794 Witnesses were Bernard Denning and James Corcoran. I did not find any baptismal records for children. Ardagh records start in 1792.
Is it possible that your Thomas Denning was older than you thought?