They're all pretty.
Iris is my favorite out of all of the names, but I'm not too sure about the way
Riona Iris flows. Then again, I'm guessing on how you're pronouncing
Riona (REE-na, REE-ə-na, or ree-OH-na?). I prefer
Lily to
Marilyn, but I think that
Riona Marilyn flows better than
Riona Lily does.
More ideas:
Riona LillianRiona LaurelRiona LisetteRiona MargueriteRiona MeredithRiona Vivian /
Vivien /
Vivienne