The Catholic Mass in the 1880s was in a number of respects quite different from today's. As other posters have pointed out, it was said primarily in Latin, known as the Tridentine rite. Also, of possible importance to your plot line, the altar was against the front wall of the church (the back wall from the outside), and the priest spent most of the time with his back to the congregation, so he might not have seen someone leave. There were key moments when he was facing the congregation, so it still could have happened. You can find videos of the Tridentine Mass on line, and most big cities have at least one Catholic church where the Latin Mass is offered if you want to see it in person. Today's Latin Mass is different in some small ways from the 1880s version (there were changes made in the 1950s and 1960s), although figuring out in what ways would take some serious research (and some knowledge of Latin). (Wikipedia, BTW, has a pretty good article on the history of the Tridentine rite.) Going on my experience with the Latin Mass in the late 1950s and early 1960s, while the Mass was said in Latin, the three scripture readings were read aloud in English and the homily was also in English (in the 1880s, this would have been in the primary language of the parish population, English, German or Italian; my grandmother grew up in a primarily German immigrant neighborhood and told me that the only languages she heard in church were Latin and German). Nobody is ever "saved" at Mass because Catholic doctrine teaches that we're all saved initially by baptism, which happens shortly after birth. Holy Communion then would have been given only the form of the host, only on the tongue, and only to congregants while kneeling at the communion rail. Back then, if someone belonged to one parish for a long time, it was not unusual for one priest to be pastor there for many years, decades even (nowadays, most dioceses rotate them about every seven years). There were a number of southern cities and towns, such as Savannah and New Orleans, with significant Catholic populations. And Savannah's was heavily Irish. Don't know if you're still working on this, but hope it helps either you or someone else.