Hello Aoife, in the way of such things, i just made the same discovery a short time ago.
I checked out the marriage of William and Bridget on rootsireland and it gave his father as Nathaniel.Still dont know how to relate Nathaniel and Thomas Todd though.
I found a Nathaniel Todd of Cappagh (28yr and a grocer) married to Eliza Darcy of Doon south in 1884, his father was also Nath. Also a Mary Todd of annacarthy 26 yrs married to Philip Doherty of C/white in 1876, her father was Nath too.
I wonder what became of Nath who married Eliza?? Would they have emigrated? and where to? I havent been able to find them in 1901 Census.
So many questions, but then thats part of the fun. Mary