Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Belfast to San Sebastian as you normally would.
San Sebastián (EAS) is the airport you will fly into if you book a flight from Belfast to San Sebastian. You should not have trouble finding flights to San Sebastián from Belfast; the airport handles an average of 0 inbound flights from Belfast per day. You should also have plenty of airline options to choose from since 0 airlines have flights to the San Sebastian airport from Belfast. It is only a 9.7 mi journey from San Sebastián (EAS) to San Sebastian’s city center.