3. Price. Fares were not cheap, by any measure
That's not entirely true. When the route was upgauged to an RJ-85 the fares reduced significantly - yet the aircraft still couldn't be filled.
I remember flying the route for about EUR 150 return.
The return of SNN-LHR was the key contributor to its demise. When LHR returned the connectors reverted to their old travel patterns. There was/is insufficient O&D demand between SNN and Paris to support regular services.