Factors:
a. Closest Airport to SJPdP is Biarritz.
b. The closest airport to SJPdP in Spain is San Sebastian, which is 3 km from Irun/Hendaye border crossing and rail service provided by SNCF and Renfe.
c. SJPdP has SNCF train/bus service to Bayonne.
d. SJPdP has seasonal CONDA (ALSA) bus service to Pamplona.
e. There is Transports64 Regional bus service from SJPdP to Pau. Pau has a regional airport.
f. Consider how you will get home from Santiago.
A. If I were traveling from Biarritz to SJPdP I would:
a. Take Chronoplus bus 14 from Biarritz airport to Gare Bayonne.
b. Take SNCF TER 62 train/autocar service from Bayonne to SJPdP.
B. If I were traveling to SJPdP by way of Paris (or Bordeaux) I would:
a. Determine if it was cost effective and time effective to fly from Paris to Biarritz. Time effective means arriving in time to catch a train from Bayonne to SJPdP on the same day. (Allow 4 - 5 hours to transfer from CDG flight arrival to Orly flight departure)
b. If arriving at CDG and taking the train is the better option ... take the train from CDG 2 to Bayonne if the schedule is viable.
c. If CDG 2 schedule doesn't work ... Take RER B train to St Michel and transfer to the Metro. Take the line 4 Metro to Montparnasse. Then board the TGV train originating from Montparnasse to Bayonne. There is more frequent service from Montparnasse.
d. If arriving at Orly ... take the Orly Val train to station Antony and RER train to Massy. Take TGV from Massy to Bayonne.
C. If I were traveling to SJPdP by way of Madrid I would:
a. Take the Metro (from either T1T2T3 or from t4) or Renfe Cercanias (from T4) to Puerta Atocha and take a Renfe train to Pamplona.
b. Or ALSA provides bus service from Madrid T4 airport to Pamplona via Soria or Zaragosa. Bus service is slower than the train however it runs frequently, is cheaper, and requires no knowledge of Madrid transportation systems.
c. CONDA provides (seasonal) bus service to SJPdP - See ALSA schedule - from the Pamplona bus station.
d. I would consider a flight from Madrid to either Pamplona or San Sebastian.
D. If I were traveling to SJPdP by way of London (Dublin would work too) I would:
a. Tranfer from arrival airport (Probably Heathrow or Gatwick) by National Express bus to either Stansted (Ryan air has flights to Biarritz all year) or Gatwick (Easy Jet has summer flights to Biarritz).
b. Fly from Stansted or Gatwick (or Dublin) to Biarritz France.
c. Take SNCF train and bus service to SJPdP.
E. If I were traveling to SJPdP by way of Barcelona I would:
a. Take Barcelona Cercanias to Barcelona Sants station.
b. Take either Renfe train or ViBasa (Monbus) bus from Barcelona Sants to Pamplona.
c. Take CONDA bus from Pamplona to SJPdP.
d. I would consider a flight from Barcelona to San Sebastian.
F. If I were traveling to SJPdP by way of Bilbao I would:
a. Take PESA bus from Bilbao (Loiu) airport to San Sebastian.
b. Take PESA or ALSA bus from San Sebastian to Bayonne. ALSA buses go to Gare Bayonne (TGV Bayonne)
c. Take SNCF train and bus service from Bayonne to SJPdP.
G. If I were traveling to SJPdP by way of Lisboa (or Porto) I would:
a. Take CP overnight train from Lisboa to Hendaye France.
b. Take SNCF train from Hendaye to Bayonne.
c. Take SNCF TER 62 service to SJPdP.
These are but a few of the options.
Regardless of whichever city you choose to arrive in ... it will take a full day of travel to get to SJPdP.