Airlines only have a certain amount of aircraft. If EI/FR aren't serving many destinations out of Cork, it's because their aircraft will make more profit elsewhere. You can be sure EI and FR look at where people who use their aircraft are coming from when booking. If they see a certain percentage of their pax flying out of Dublin to wherever are from Cork, they would be tempted to serve that destination directly. However, they have to see if
A) Would the new service be profitable
B) Would the Dublin service still be profitable without the Cork pax
C)Would the aircraft needed to run this service be making more profit than if it were used on another route
If the above are all met then maybe there is a chance. Maybe
Same goes for SNN, and given they can set their own charges, they are Cork's biggest problem now IMHO, as the FR move of the Polish flights demonstrates.