to the op - i see tha you're from Ireland, therefor i would say to you that wherever you live in Berlin (ok, bar the really bad areas like Mahrzahn) you're going to be a lot safer than in DUblin so i wouldnt get too caught up on p-berg versus F-Hin versus
Kreuzberg. I'll probably be shot down for saying this, but they are all variations of more or less the same thing IMO. Trendy - perhaps, but not in a Dublin kind of way... i.e. you're not going to see tonnes of girls with bleach blonde straightened hair and ugg boots and Daddy's credit card hanging out of rugger-buggers...trendy in Berlin is relative, by that i mean its arty rather than trendy london or trendy dublin where the scene is much more money driven. Trendy in Berlin is more like an NCAD gathering

Its all good in my opinion. i like getting away from the money-hungriness of dublin so the arty kids dont bother me.
I agree with people who said that there is waay too much labelling of areas going on... an to the nth degree -down to streets and squares. however, seeing as you may not know the city that well ill summarise it for you in local terms

Mitte - the Ballsbridge/Baggot st/ranelagh of Berlin. nuff said.
P-Berg - the liberties - used to be rough but now is full of young professionals.
F-Hain - bit like thomas st area of christchurch -cool pubs, bit rough but overcompensates by the nightlife
Charlottenburg etc - Rathgar. Boring as hell but really safe. Old money. I stay there when in berlin, simply cuase i can easily get to f-hain for going out etc, but its safe and within walking distance of k-damm.