The flight plan can be filed while airborne.
However having said that, I've never been refused a transit through Dublin a/s, and I always file a ICAO plan while on the ground. I've often heard of people who tried to file on the radio being refused, as ATC was too busy to take the details over the radio.
I'd suggest you you file a paper one.
edited to add. If you are departing a controlled airfield, they will not accept a f/p over the radio. It must be filed with AIS in Shannon at least 1 hour prior to departure. The new number is in the notams.