So I've done a little more research (coupled with paying just a little more attention) and...(note the following is based on accepting the name exclusively means little king...rí meaning king, similar to rey etc in spanish and rex in latin)
"The suffix -án is the Old Irish form of the dimunitive -ín"
So yes it would make sense and having looked at Ciarán etc (which I shouldn't have dismissed out of hand) it is very clearly being used as a dimunitive. And Ciarán etc is correctly spelled only with the fada. Frankly I think I am now the second most interested person in this question over the correct spelling, after you!
So if I was to suggest a correct spelling now I would go with Ríán. (There is currently only 1 person I can find on google with that name atm, presumably his mum would know, lol) .And based off this I would never suggest spelling the name without a fada on the "a". So does it just come to whether it is Rián or Ríán...but I am basing all this on accepting the given explanation that it means little king. But...'According to John Ryan, Professor of Early and Medieval History at University College Dublin, What the Rian in the surnames Ó Riain and Ó Maolriain is has never been satisfactorily explained. Rian, like Niall, seems to be so ancient that its meaning was lost before records began'.
We also need to make sure "O'Riain" isn't simply the anglicised version of "Ó Ríain"
I also came across this..."One old Irish word for the sea or ocean, as here, is rían (< Proto-Celtic *rēnos "river, waterway", from which stems the original name for the Rhine river). The use is all but gone in Modern Irish, though, except maybe in the phrase gabhra réin in FGB ("sea-mares", white-crested waves, that is, white horses)."
God will know what the story is, I'm just not sure who else will😆