I think by far the biggest irony in Act IV scene 1 of this great play is the way that Portia manages to "win" the case against Shylock and save Antonio, her husband's friend. She does this by at first initially indicating that Shylock is completely right - the bond that he and Antonio made means that he is perfectly entitled to cut from Antonio "his pound of flesh":
A pound of that same mercfhant's flesh is thine.
The law allows it, and this court awards it.
To this announcement and to others like it, Shylock responds with praise of the judge: "Most rightful judge!" It is extremely ironic, however, that in spite of his praise, this same, "rightful judge" will also be the means of his defeat. For, by following the strict letter of the law, Portia makes it impossible for him to claim his "pound of flesh":
This bond doth give thee here no jot of blood;
The words expressly are "a pound of flesh."
Take then thy bond, take thou they pound of flesh;
But in the cutting it if thou dost shed
One drop of Christian blood, thy lands and goods
Are by the laws of Venice confiscate
Unto the state of Venice.
Having praised the judge, Shylock is now faced with the way that Portia has tricked him into an impossible situation. By insisting on the law being fulfilled, he is definitely getting more than he bargained for because of the exact fulfilment of the law that he has desired. This is clearly one of the most profound ironies in this scene.
If you want to find some more, you might think about the interplay between Portia and Nerissa and their husbands, who of course do not know their identity.