Co. Louth rcds are for only RC at present. I'm not whether there was a significant protestant presence in Louth. The counties to the north of Louth had protestant majorities, although protestants only made up abt 12% of the population overall.
You need to practice using the database. 1st, unless a specific county is selected, you get results for all Ireland. 2nd if you specify one or more parishes, you get results only for that/those parishes. 3rd when you click on the magnifying glass, you get rcds for that ONE county, and no other. 4th, within that specific county, you have the option of restricting your search to a specific parish. As long as you don't change the parish name, you continue to get rcds for various surnames; however, you have to specify a forename. Generally for the forenames, I just insert the first letter for the forename. It then returns all forenames beginning with that letter. This is a tedious, repetitive process. If you want a specific forename, I just type the first 3 letters for it.
If you don't have specific info, viewing detailed rcds gets expensive fast. It really helps to have a relatively precise DOB (at least the yr), parent(s) name(s), and/or placename. If all you have is a "Patrick Kelly born in Ireland," the database lists about 7,500 rcds. This puts a premium on getting as much info on your ancestors as possible from U.S. sources, especially the oldest living members of your family line.
Generally in a mixed marriage, the bride followed the husband's religion. Both the bride and groom were promptly ostracized by their parents. This is still true to some extent today. The catholic family often looked upon their grandchildren as being illegitimate.