Me and my mom will be arriving in Madrid at 05:00 on Good Friday next year. I need to go to Irun and she is on her way to SJPDP. I have a feeling we might struggle with bus/train availability over Easter Weekend. Does anyone know how big a problem this might be?
I am struggling to understand the Renfe website and ALSA also doesn't seem to have timetables for April yet.
If I were you, I'd consider getting a flight from Madrid to San Sebastian EAS or Biarritz BIQ.
EAS airport is about 3 km from Irun and Gare d'Hendaye.
BIQ airport is 40 minutes local bus ride from Gare Bayonne and is the closest airport to SJPdP.
SNCF train from Hendaye to Bayonne and Bayonne to SJPdP has TER service which there are no reservations required or available.
ALSA/CONDA likely won't have anything posted more than a month in advance. RENFE schedule is kinda sorta 3 months for its AVE but sometimes much less for its less important routes. SNCF is usually available 3 months out and doesn't usually change much so you can plan using tomorrows schedule and not be far off when its time.
I wouldn't count on there being bus service from Pamplona to SJPdP in April. It might start running that early in the season but no guarantee that it will.
Easter is a big deal in Spain and France ... so expect long lines and delays if you don't have reservations and tickets when you get there.