Belfast's airports don't really have any competition for passengers in Northern Ireland. City of Derry is there, but doesn't offer too many routes.
Cork is competing with Shannon and to a lesser extent Waterford, Kerry and even Dublin.
Perhaps the most realistic comparison is between the population of Northern Ireland (1.7 million IIRC) and Munster (1.2million) as potential catchment areas and then remembering that Cork shares that Munster catchment with Shannon.