Normally Midleton and Cobh services are provided by 2-car DMUs. All platforms can take 4 cars without any alteration. So the capacity of the existing hourly service could be doubled without extra fixed assets, or quadrupled by moving to a 4-car 30-minute interval service. And I daresay that extra peak hour services might easily provide even more capacity.
For all of this you will need more rolling stock. More track capacity would be a total waste of money. Services in the Mallow direction are another matter.