Hi Okrad
I have had much of the same problem as yourself regarding RC Irish Immigrants coming to Port Glasgow. It's actually been my brick wall i've been up against for many years. And with names like Kelly & Devlin, it's been like a needle in a haystack lol. To this day I am still unsure, although Inishowen seems the most probable in my case, if you like you could post the family names and maybe myself or other rootschat members could offer more information in helping with your search.
Kind regards
Fiona