The definition of the catchment area for something like an airport is:
the area and population from which a facility or region attracts visitors or customers
Now, the last time I flew into Derry I didn't see many people living on Lough Foyle, but there you go.
LDY has a city of over 100,000 people seven miles away, and a town of over 24,000 and several with populations over 10,000 within 20 miles.
Imagine how busy NOC would be if it had local urban areas like that.
As an aside, and slightly off topic, what impact, if any, will the completed M6 have on NOC? Dublin in a few hours?
I ask as the infrastructure in the NW is slowly improving with new dual carriageways either recently completed, or in the (very long) pipeline.