No luck finding Dennis & Catherine's marriage on rootsireland or findmypast.ie but fmp has a few Dennises born around 1820 which might be worth a look (credit required).
There are 78 Hughes in Armagh in the Tithe Applotments in 1827 (free at http://titheapplotmentbooks.nationalarchives.ie/) & 837 in Armagh in Griffiths Valuation 1848-64.
Armagh is not included in the Irish Censuses of 1821, 31 or 41 but there are 3 Hughes in Armagh in 1851 (free at http://www.census.nationalarchives.ie/)
No luck on ancestry or Family Search either.
If you google PRONI and select Name Search then enter Hughes Armagh and tick all the possible records you'll get 67 listings including a Denis in 1801 in Corr Armagh in the Pre-Admin Wills