Hi Jen,
It's so frustrating when you can't find anything, but I can't give it up, one day as more and more goes on the net something may just turn up, so don't give up hope on finding out about your "Sheehan" ancestors.
I've just been looking back over the material I have collected since searching Waterford there was a Patrick Sheehan (Labourer) living at 22 Blakes's Lane (off Barrack St) Waterford City 1909/10. I don't know if he is one of yours but every little bit of info could be important. I posted a message on the Waterford board with a link to the site where I obtained this info, I'm sure you would think it worth a look as you may find more of your family.
Jan