Why is everyone looking straight past Cave? He's been on really good form, very good defensively and is quite quick. He's decent size and has good hands. You're really suggesting Downey over Cave?
12: Cave / O'Driscoll
13: O'Driscoll / Cave
Both players have played in both positions - I think it could work.
DC hasn't played much at 12 this season, maybe 1 half IIRC. I'd also say his style is too similar to BOD to complement it.
Downey has looked alright for Munster this year - and we've seen in the past how BOD works outside a hard runner (Lions with Roberts).