DUB was always going to grow first, that's just the nature of being the business, population and tourism capital. If you had said to me SNN would grow before ORK 2 years ago I would have thought you were mad. But SNN being cut loose from the DAA changed everything. Shannon, Kerry and Cork are all competing for the same passengers. In a purely commercial environment (ie no state ownership or subsidies of any kind) it's very unlikely all 3 would survive.
We are left with the legacies of previous aircraft requirements and political interference as regards where our airports are. If you were to start from scratch, Dublin would have 2 parallel runways in excess of 3000m with 1 terminal where T1 is and another where the tower and Cityjet/Aer Arann hangars are. Cork would have 2 intersecting runways, one of which would be in excess of 3000m (in a SW/NE direction of course) and SNN would be a regional a la Knock, but probably state owned, and Knock would be as it is now. That's plenty in a country with a proper road network