My opinion: The umpire cannot call the IFR after the fact. Because it is a judgment call (If in the impires opinion the ball can be caught by an infielder), there is no IFR if it is not clearly called by the umpire with the ball in the air. Since it had not been called, the ball is in play, nobody is automatically out, all runners are forced.
There was an article in Baseball Digest, that reclounted four such incidents in the Major Leagues
Baseball rules corner: many players unaware of tag requirements when Infield Fly rule is called | Baseball Digest | Find Articles at BNET.com
but the article did not address the question of whether an umpire can call the IFR in retrospect, after all the baserunning and tags have been completed.