In Hamlet, when Ophelia tells his father that Hamlet is mad, how does the dramatic irony affect the plot, mood, theme and character? Hamlet, Act...

In Shakespeare's Hamlet, Act 2.1,
when Hamlet accosts Ophelia as he does, the themes of madness and appearance vs. reality
are probably most directly revealed.

Ophelia breaks off
contact with Hamlet, setting up the sequence of events concerning her relationship with
Hamlet.  She is acting, and will continue to do so later when she returns Hamlet's gifts
to him, spies on him, etc.  Hamlet is acting--pretending to be mad.  And behind it all
is Hamlet's plan to make Claudius become preoccupied with the reason for his madness,
and Claudius's attempt (also Polonius's obsession) to find out whether or not Hamlet is
mad and why, and, as far as Claudius is concerned, whether or not Hamlet is out to get
him.

Basically, everybody in the play is acting, and this
scene just features Hamlet and Ophelia doing it.  No one is as they
seem.

Concerning the one topic you leave blank, if you're
using mood when you mean tone, what we have are Ophelia's words describing what
Hamlet did.  Thus, we are looking for Ophelia's tone as she relates Hamlet's behavior. 
I suggest her tone is probably a frightened or panicked tone.