SJPdP is connected to Bayonne by SNCF train.
Bayonne is connected to the rest of France by SNCF rail.
Biarritz BIQ is the closest airport to SJPdP. There are flights to Biarritz from several major cities in Europe (including Dublin and Madrid).
SJPdP is connected to Pamplona by CONDA (ALSA) bus
Pamplona is connected to the rest of Spain by Renfe train and several bus companies.
Pamplona has an airport PNA with connections to Madrid.
San Sebastian Airport EAS is the closest airport to SJPdP in Spain. Its located about 5 km from the terminus of the SNCF trains in Hendaye. There are flights from Madrid to EAS
Santiago is connected to Madrid by Renfe train.
Santiago has an Airport SCQ with flights to several major cities in Europe.
There are discount airlines in Europe ... Ryanair, Easy Jet, Transavia, etal. They sell low cost seats on one way flights making it easy to get around in Europe.
Aena manages all airports in Spain. See their website to find destinations and service providers.
Your plan needs to include details on how to get to either of Bayonne or Pamplona (if SJPdP is your planned starting point) and how to get home from Santiago.