Hello Josie,
I looked at the Valuation extracts on John Hayes failteromhat.com site. There was no sign of Denis Cannon but here are other counties where there was a guy of that name.
Denis Cannon - Griffith's Valuation - 1848-64
Donegal 1857
Denis Cannon, Dunmore, Mevagh
Denis Cannon, Ballybrollaghan, Inver
Denis Cannon, Brinlack, Tullaghobegley
Denis Cannon, Clonbara, Tullaghobegley
Denis Cannon, Kilrean Upper, Killybegs Lower
Kilkenny 1849-50
Dennis Cannon, Fishersgraigue, Graiguenamanagh
Wexford 1853
Denis Cannon, Town of Wexford, St. Peters
Christopher