are you looking for their irish background? You don't give a year to start:
Tithe Applotments 1823-1837 now used as census substitutewww.askaboutireland.ie
Griffith Valuation 1836-1864 now used as census substitute
look for both surnames even if left earlier as families generally were clustered, look for common parish for both surnames.www.census.nationalarchives.ie
Directories 1824 & 1846 (farmers don't show till 1881 & 1894)www.igp-web.com
+ county generally a good resource depending on the countywww.rootsireland.ie
parish birth and marriage records a paywebsite like most of Irish genealogywww.familysearch.org
civil registration births and marriages 1864+ , death registration has people who died after 1864 but some born in late 1700's.