Hi Lee, I would not rule out a Catholic marrying a Protestant, not unknown but unusual that the marriage was not Catholic, while researching my own ancestrors there was a marriage pre Catholic emancipation in 1829 with a long note to the entry that it was a 2nd marriage for the couple in a military Protestant church as he was to be posted overseas and she was securing his pension rights which were denied if you did not marry in the Established Church, think this was gone at this point but they may have been concerned, what religion were the children reared.
There are no Stacks in Griffiths Valuation but if the parents lived in the barracks they would not be renting or hold land, have you looked at the military records to see if there are any Stacks there.
Also there are a number of private Facebook pages doing either Gedmatch groups or matchmaker tool using Gedmatch, I am on a few and while they are interesting and well moderated people tend to join too many if they are not sure where they are from, however some are more localised and there are at lest two national ones, most people though are lost but your match is close enough to possibly get a better match, if you are not on Gedmatch read the terms and conditions etc and I think all disputes are settled under Israli law.
I see two Connell families in Griffths in 1851, both in the town of Fermoy, a Thomas and a Michael, parish Glanworth, again just shows the family name there but it is not an uncommon name.
Good Luck
Pat