1,000 ft would increase the number of aircraft types that could use Cork for transatlantic (important considering the 757 and 767 aren't being made any more) and also extend the range to include cities like Orlando and Chicago (not sure if it would be enough for West Coast) or Dubai to the east.
Obviously, it would still be shorter than Dublin's inadequate runway, but that's inadequate considering potential demand for flights to places such as Hong Kong and the need to take in larger freight aircraft. I doubt large scale freight or Asian destinations are things that Cork needs to worry about.
Most particularly, going more than 1,000ft would mean either very expensive land levelling to the south or some form of extension to the north. The cost of a southern extension couldn't be justified on the grounds that it might be needed. Completing a northern extension at the same time as a southern extension would result in negligible cost savings, so it may as well be done at a later date.