Fair assessment, at present you have to go with the more experienced O'Shea, however why not give St Ledger some games in friendlies to help bring him into the set up.
Heres a scouting report of the lad, may give you some idea of his qualities, note his ability to read the game, something McShane will never have and is priceless at the top level!
http://blogs.guardian.co.uk/sport/20...st_ledger.html