Sorry I don't know specific times, but I am sure you must be able to get there within a day. If you get on a fairly straight-through train that is going through Lourdes via Bayonne or Dax to Hendaye-Irun, that is only about a 2 and a 1/2 hour trip.
Last year I wanted to get from Moissac, in the south of France, to Pamplona, and I did it within a day- with four trains. I went from Moissac to Toulouse to Bordeaux to Hendaye-Irun, then Pamplona. When I finished in Santiago last year I caught a bus that finished in Irun, then caught the little train that takes you across the border to Hendaye, then got back to Moissac via my three trains in reverse.
Basically, if you aim to get from Lourdes to Bayonne, you can get a train to Hendaye, then it's easy enough to get to Irun. I am not sure of timetables from Irun to Pamplona, but I was able to get a train that left Irun mid-afternoon or so, and I got into Pamplona about 5.30pm from memory. (The train was continuing onto Barcelona I think.) Buses leave from right next door to the train station in Irun.
Margaret