Cork had little or no serious infrastructural investment from 1961 until the building of the new terminal. The old terminal was inadequate once passengers passed the 1m mark and was more suited to the Viscount era than the 21st Century.
The old terminal had simply outlived its usefulness and no amount of minor extensions and refurbishment was going to change that fact.
The road structure, Car Parks, drainage, apron etc also needed to be upgraded. The one criticism I would have of the DAA is the amount that was spent in making the new terminal look visually attractive.
I would also have doubts about its ability to efficiently handle 4m to 5m passengers in its current form but that's for another day.
Cork like Dublin experienced rapid growth between 1995 and 2008 when traffic quadrupled from 800.000 to 3.25 m. Two years of consecutive growth of 12% will take it back over 3 million again.