I wouldn't close any regional airports, just not give them capital funding for expansion.
Realistically, you'd be looking at maintaining the three large airports and developing one of the north-western airports to provide a decent centre in the region. For many reasons, Knock is the best choice there. It already has a decent length runway. Galway is too close to Shannon. Sligo is unexpandable without going to great cost and environmental damage. Knock's location isn't ideal, but it's not the worst either.
If you were going back to plan aviation from scratch, you probably wouldn't even have Cork and Shannon in their current forms, just a single south-western airport located somewhere between Mallow and Croom, with 7 million passengers and decent rail and road access to both Cork and Limerick as well as Kerry. But that's an option that's long since gone. In the north-west, there's still the opportunity to get it right.