Northern Ireland, Derry, for three weeks

Hey I have a couple of questions if anyone has been.
I'll be staying in Derry (inside the wall if that matters) for two weeks, then probably taking a tour of the rest of Northern Ireland for a week.

I am staying in a hostel which includes breakfast. Beyond that, I have a $20/day budget for food/drinks/entertainment. (Tight, I know, but it can be done!)

1. What are the bad neighborhoods/ streets in Derry I should avoid?
2. What should I definitely see in/around Derry? (I'm on a $20/day budget)
3. General recommendations on healthy eating at a hostel? (tight budget, not an awesome cook)

I think that's it but I'd love to hear anyone's experiences in Northern Ireland.