It actually used to be like that, but the process was open to abuse because Club X would say - oh, well that means that this player will miss a midweek club game on Tuesday and another on Thursday and so he'll be free to play again on Saturday. »
I wouldn't rule out any of England or Ireland, even Wales. The world cup knock-out games are one-offs, and strange things can happen. England's forwards can do a lot of damage, and Ireland may have the best flyhalf in world rugby. Can make all the difference in a one-off game.