Hi There,
The best we can do at the moment is say that Robert's parents were Mr. & Elizabeth Robinson.
I am working with the presumption that the family aren't originally from Rallagh (I could be wrong on this though)
Ellen Smith's father was called Andrew Smith and was a farmer and appears on the GV in Rallagh.
Robert states that his father was also a farmer which means he most likely should appear on the Rallagh GV if the family were there at the time but the nearest is a John Robinson another townland but gut instinct is saying that isn't him.
I have look at pretty much all the male deaths for Robinson in Limavady up until 1877 when Elizabeth died but I don't spot any good matches.
He possibly died prior to 1864, when records started.
This may be when the family moved to Rallagh . . . Possibly Elizabeth was originally from there . . . Who Knows . . .
The stumbling block is that none of Robert's siblings, who we know for now, got married, so we still don't know the Christian name for Mr. Robinson.
So, as it stands we don't know Mr. Robinson's Christian name OR where the family originated.
You have asked how to take your tree back further, well for now you can't . . .
I know you found it tempting to look at a local parish and think that David & Jane Robinson in Binn were your ancestors but they aren't . . .
I think, for now, you have to be happy that at least you can clean your tree up and remove all the Binn Robinsons and add all your actual ancestors the Rallagh Robinsons . . .
I am sure that if anyone spots any further leads they shall post them . . .
Tara