If Shannon closed, you could be sure someone would buy it and reopen it. The airfield facilities are fantastic and it has a reasonable catchment area. The main problem is being stuck with a cost base from when it had more significance. But if you bought the facilities, all those disappear.
I suspect we will see the closure of some Irish regional airports when the PSO routes are scrapped.
Sligo seems particularly vulnerable as it is so close to Knock (as pointed out elsewhere, it takes less time to travel from Sligo to Knock than it does from some parts of the southside of Dublin to Dublin airport) and is constrained in terms of expansion.
Galway has similar problems. A larger catchment will earn it a stay of execution, but with the future motorway making it even easier to get to Shannon, I would be surprised if it is still around in 20 years time.
Dublin, Cork, Shannon and Knock are safe. I reckon Waterford can be made to work. Kerry can probably make some kind of future too if it can concentrate on inbound tourism.
I have no idea about Carrickfinn. In some ways it looks the least attractive, but it's also a candidate for PSO retention considering there are no other reasonable public transport methods of getting to Dublin.