The difficulty with Ireland is very few parish records this early.
I would try searching for Henry Knight 1801 as your best opportunity. Both Fermanagh and
Cavan belong to an Irish History Foundation www.rootsireland.ie
a birth record would give you the parents names, although you could take a good guess as you will find the father' and mother's names in all the siblings children.*
Henry was unusual in those days, also Samuel (very rare) Richard not common either, nor William.
Thomas more common and James and John were the most used forenames in Ireland, along with
Patrick and Michael. This could possibly help you find the family. William is a CofI name most of the time.
Butler's were everywhere the highest concentration in Kilkenny.Tamzen is a woman's name, don't this that very often, even today would be unusual.
The earliest census substitute for Ireland is the Tithe Applotments 1823-1837www.genealogy.nationalarchives.ie
No Tithes for Fermanagh, at least no Knights
1 Knights Thomas, 1833, Robert 1832 in Cavan.
Next census substitute is the Griffith Valuation from 1846-1864 (different for each county)
This is late for your family but unless a complete family moves there would always be those left
Cavan has 7 Knights listed in 1857: William; Joseph x 4 including 1 owned property; Abagail
Fermanagh has 2 Knights listed in 1862: James; Thomas both of Clones Parish.
*Fermanagh records has 1 parish Drumglass that has records from 1783-1790 & 1821-1900.
Cavan's records currently being updated
The Stewarts had a heavy presence in Cavan, originally from Scotland - came/brought as farmers and by 1862 there were only 30 listed
Gould's were not big any where, but 6 in Fermanagh and only 0 in Cavan; Dewhurst only 2 in Lough, and perhaps a different spelling would change those numbers.
Index of the Registry of Deeds-Ireland (freepages)
William Knight, Lisnenenagh Cavan and property at Breaghdarrig, Cavan
Good luck in your search.