Aer Arann
Of course Ryanair's weather record is far better than Aer Arann on Dublin Cork. In almost 14 months not once has a FR domestic flight been diverted to SNN or KIR although they did on a handful of occassions return to Dublin.
Ryanair are CAT2 compliant and more importantly avail of this facility when the Cork RVR goes below CAT1 minima.
Cork Derry could work for Aer Arann but the amount of diversions and cancellations on Galway Cork particularly on Sunday's is on the high side.
Cork Luton sounds OK but at the end of the day it's in the London catchment area and that means competing with Ryanair and Aerlingus.