how to get to SJP from either Madrid or Barcelona

Renfe train from Barcelona or Madrid to Irun/Hendaye. Transfer to SNCF train from Irun/Hendaye to Bayonne. Final leg is SNCF TER (#62) Bayonne to SJPdP.

It would also work going north from Barcelona on the new train line to Paris and connect to Bayonne using SNCF. I haven't worked this route out so I don't know the transfer points.

Renfe
http://www.renfe.com/EN/viajeros/index.html

SNCF
http://www.sncf.com/en/passengers

SNCF TER
http://www.ter-sncf.com/Regions/Aquitaine/fr/Default.aspx