until paternal blood/dna tests it was a wise child that knew its father! - presumeably you have obtained birth certificates from general registry, Douglas; if these don't name father or name another then I see little chance of proving paternity other than by association - voters/rate payers lists for Ramsey may help prove possibility (see General Registry - not on-line), if putative grandfather was a mariner then the crew lists now available at Manx Museum may prove when he was off the Island, have you checked local paper (Courier) of period to see if they are mentioned (eg any court case etc ?).
If you are rich and desperate enough then by locating known offspring it may be possible to prove that someone was not the father but I believe private DNA tests (assuming others are willing to provide samples) are relatively expensive.