Have you the names of Joseph'e siblings at all? My GG-Grandmother was Margaret Flanagan (b. c. 1833 / m. James Moran 1858). Margaret had four siblings we are aware of - Eleanor, Patt, Ellen and Michael. Even though she would have been much younger than Joseph, that could very well be given the huge families (James had a sister 20 years younger)they all had.
Also, James had a sister, Mary (b. 1835) who married a Michael Duffecy in 1865. We also have another Moran/Dufficy marriage later on in the next generation. These folks didn't move around very much, even on a day-to-day basis.
These families all lived in the townlands between Elphin and Tulsk, namely Currabaun and Clooneyquin.
Hope to have helped,