dublinpilot
I am sorry to labour this point but it interests me because I want to go there again someday (been to Waterford a few times, VFR)
If you file an ICAO FP from A to B, and there is some CAS between the two (but not overlapping either of A or B) then the ATCU for the CAS will not get a copy of the FP. The FP will (by default) go to A, to B, and perhaps to the regional Flight Info service (London Info here in England). This is my understanding. I believe that if one is smart one can dig up the 8-character addresses for other places and get it addressed to additional units.
So I cannot see how the filing of a FP helps with Class C/D transit, unless the ATCU for that CAS is one of the addressees for the FP.