What is situational irony? How does Antony show it at the end of the play?

Your question concerning Shakespeare's Julius
Caesar is complex, and sometimes determining types of irony used can be
difficult and can force one to make decisions along a fine line, so to speak.  Also,
your question is a little vague.  I'm assuming you're referring to Antony's final
speech, in which he praises Brutus as noble, pointing out that Brutus assassinated
Caesar because he thought it was the best thing for Rome, unlike the other
conspirators.

Situational irony involves a
difference or an incongruity between expectation and reality, and derives from events or
situations, not from statements made by a
character. 

Thus, Antony's words do not in
themselves constitute situational irony.  They may, however, further reveal and
highlight the situational irony present throughout the play:  that actions stemming from
noble motives and a noble cause can lead to an ignoble ending, to tragic consequences. 
 

It is ironic that although Brutus acts because of reasons
that are noble, he causes a civil war and ends up dead, as do many others.  His
motives are pure, but the results are tragic.  If
expectations are that noble Brutus will be victorious, the
reality is that he isn't.  Antony doesn't directly state
the irony (he states only that Brutus was different from the other conspirators), but
that is irrelevant.  Characters do not have to be aware of situational
irony. 

Related to the above is Brutus's thinking that his
ability to judge people and to make decisions is good enough to pull off the
assassination and maintain power.  His misjudgments and poor decisions recur frequently
in the play.  He thinks--he has expectations--that he is a
good judge of character, and a good decision maker.  Others think so, too.  The
reality is that he is neither.  Nobility and pure motives
are not enough.  Brutus thinks he can lead the conspiracy to victory.  But he doesn't.