Quote airnoc "What would CAT 111 cost to install now that CAT 11 is already there at Ireland west airport Knock?"
ILS operations require a whole series of systems working together: runway surveillance systems, taxiway/guidance/approach lighting, fire cover, staffing (ATC operation and systems testing/monitoring/training, runway inspection and security), insurance and legal requirements, along with the obvious ILS antenna, backup and power systems. I believe EIKN has CAT II approach lighting which would need upgrading along with accompanied civil works which would be a significant cost to start with.
That's only the ground element, the airline would need to have CAT III equipped and serviced aircraft as well as regularly ILS rated crew. RE don't rate their pilots, WW rarely perform a CAT II landing and I'd imagine Ryanair probably don't have to do many CAT IIIs so may not pay for rating pilots. All in all this means CAT III is very rarely justified at small airports, and often not available at many larger. Even DUB had many weather diversions this week.
The CAT II upgrade seems to have addressed many of NOCs low cloud base diversions with very few in 09. This winter has been historically long and cold leading to ice and freezing fog which along with snow closing airports in the UK means cancellations are inevitable.