The problem for TK is that with DUB already a successful station by adding additional flights they offer additional flexibility and connecting oppertunities by having a choice of arrival and departure times and, potentially, shorter connecting times too. This will help to drive both volume AND yield. By introducing a flight to Cork (or Belfast, for that matter) they loose the benefit of more attractive schedules and, potentially, dilute yield as passengers may just choose the cheaper departing point. There are alos additional costs with opening the station, advertising and promoting, operating two, as opposed to one airport in a country and so forth.
It is a shame, because TK is widening their network and offering ever more connection possibilities, but it is hard to see why new airlines would come to Cork when there is a strong history of being driven out by Harps or Shamrocks.