Irony is a divergence between literal and intended meanings. One subset is dramatic irony, where dialogue means one thing to the character speaking it, and another to the audience hearing the line. The classic example of dramatic irony is Oedipus Rex, where the audience knows what will happen to Oedipus long before he does.
The most recent episode of Fringe offered another possible example -- the Observer's line that "It must be hard... being a father." The intended reference was presumably to Walter's dilemma as Peter's father. But I suspect the dialogue will take on a whole new meaning by season's end.
Many, myself included, believe that Bolivia is pregnant with Peter's child. If so, the Observer's line may be even more profound than he realizes. Specifically, Peter and Bolivia's baby may be yet another "unforeseen consequence" of the Observer's saving Peter as a child.