I research the Irish Family History Foundation's database. It contains 12,800,000 surviving rcds for abt 70% of Irish counties. Rcds are increasingly sparse B4 1840, rare B4 1800. Cos. Derry and Donegal rcds were recently added to the database.
Note 1 - Without a KNOWN county, the chance of an ancestor's rcds NOT being in this database is AT LEAST 30%.
Note 2 - Only 1/2 of Co. Cork's rcds are in this database.
Note 3 - The RC church kept control over most parish rcds from Tipperary (north). These are ONLY available thru the Tipperary Family History Research center at a substantial fee (regardless of outcome).
I searched for a marriage in Cavan for Pat(rick) Riely and Ann Smith in the range 1850-1852. It is doubtful that they would have married earlier. I couldn't find instances where there was a M rcd for each in the same yr and same parish. You can extend the search range to earlier years; however, this would likely have resulted in children being born in Ireland. I suspect the two married in America.
Suggest you use the residence noted in the 1870 census to begin a search for RC parish marriage records. These rcds, if they survived, should have listed their parents' names and perhaps placenames.
Google the Foundation's name to lead you to this database. Register, opt to be notified as rcds are added and browse. Pls post your search results to my genforum msg.