In Romeo and Juliet, can you please tell me if this quote is a dramatic irony and why? 'Tut, I have lost myself, I am not here, this is not Romeo,...

Dramatic irony is when the audience knows something the
character doesn't (the classic example of this occurs in Oedipus
Rex when the audience is aware the Oedipus has killed his father and married
his mother, but he has no idea that that is the case and vows to root out Laius'
murderer and exile him).  Dramatic irony helps the audience appreciate and understand
misunderstandings and dramatic situations in the plot of a
play.

Technically, the quote you provide is "dramatic"
irony, since it is an ironic statement and occurs in a drama; however, it is not a
classic example of dramatic irony.  Clearly, Romeo has not lost
himself--he is himself.  And he is there, the audience can see him standing there.  But
the audience and Romeo are both aware that he is not behaving or feeling as he usually
does, and therefore Romeo is "not himself." Shakespeare is playing on an idiomatic
expression, so this is not dramatic irony though it is verbal
irony.