Right. That's why I posted the earlier data, from County Down. They may not be the particular Roys you're looking for, but they help to indicate where your Robert Roy was most likely from, as they could have been brothers or cousins, or one of them might even be Robert's father. In this case, the search suggests Dromore Parish or Downpatrick. It's a place to look, anyway. Since you know they immigrated in 1847, I'd try checking the census of County Down for the nearest year prior to 1847, if there is any existing. I can't seem to find that right now, though, but, the last of the men on the list I posted earlier (John Roy, Kinallen, Dromore Parish, Tithes 1834) indicates yet another place to look, in Kinallen, Dromore Parish. It's not unlikely that Robert lived nearby. I don't know if there is a census for prior to 1847 that is still extant, so you may have to look in other records, like the tithe lists, parish records, etc. But, at least now you have a target area to search.
Gary