I finally have an answer for this. The Robert Gelling born about 1802 who ran the Paper Mill at Ballamillaghyn matches the Robert Gelling bapt. 5 Dec 1802, eldest (I think) son of Robert Gelling the Miller and Anne Kelly, and therfore grandson of John Gelling d 1812 of Ballamillagyn.

I had a note attached to the 1802 son Robert that he "apparently left the Island by 1836", which clashes with this being the right fellow. I can't relate that little piece of information to anything, so I think it may be wrong.