FWIW, I have been to Rosapenna about ten times now, and I have never flown into Donegal airport. Between the connection time, the flight time, and the 40-minute drive from there to Rosapenna, it isn't going to save you any time, and as Donal says, those flights on little planes to little airports are sometimes delayed.
If you know the route well, it takes about 3.5 hours from DUB to St. Patrick's, and you really never see traffic heading north from Dublin or back to the airport.