I don't have any idea of the costs involved to upgrade Cork to Cat 3 status, i can only use the example of Leeds Bradford as an example of a similar airport, of similar size, which also has topography issues surrounding it.
Leeds 2.9M pax Cork 2.3m
Similar mix of airlines serving with Cat 3 equipped aircraft. EI,FR V Jet2,FR.
Both have a history of fog related disruptions on a large scale.
So all i'm saying is that surely the business case for Cork would be similar to Leeds, based on the similar volume of passengers.