The Merchant of Venice is a great comedic and tragic play about the rule of law, the rules of love, the rules of religion, and where mercy has its place in all. The people in the play are decidedly antisemitic which provides, in our time at least, an added healthy dose of irony. In Shakespeare's time, did they get the irony? Did Shakespeare himself intend it? 
 I think it's likely that Shakespeare purposefully used irony, but maybe not as powerfully as we see it now. He masterfully goes back & forth blurring lines between a villain and a hero. You can't help but feel sympathetic toward the villainous Jewish money lender Shylock and some disdain for the apparent tragic hero Antonio. Modern audiences can certainly understand why Shylock seeks revenge against Antonio. Both Antonio and Shylock come off as tragic figures. 
 Shakespeare toys with the characters and the audience using caskets, masks and disguises that warn of things not always being as they appear, in love, in the law, or in religion. 
 ****Mild unspecific spoilers follow***** 
 (view spoiler)[... 
 In the end, things come together as a bit more black and white with a cunning hero who proposes a solution that probably appeared merciful at the time. A modern audience can see that the ending is hardly just. Still, Shakespeare, whether intentional or not, drew attention to the antisemitism of the time that has hopefully sparked great discussions throughout the centuries. (hide spoiler)]