The timetable is on this page:
The information is only in Spanish and Basque languages. Note that the timetable refers to 'San Sebastian' by its Basque name of Donostia.
Trains leave Hendaye at 3 and 33 minutes after each hour, every day. Last train back from Donostia is at 2145 weekdays, 2248 Saturdays, Sundays and holidays in winter. In summer you will see that the trains ruin all night on Saturdays.
The Euskotren station at Hendaye is directly outside the main SNCF station. The 'Two Twins' Les Deux Jumeaux is the next station from Hendaye heading north towards Biarritz. Not many trains stop there.
Biarritz to Pau - early trains thus
0545 weekdays only change at Bayonne
0632 not on Sundays, direct
0809 daily direct
1238 from BAYONNE daily. Connecting train from Biarritz at 1145 but that doesn't run on Saturdays and Sundays.
Reservations are not needed on these trains but you should check details for the exact date of your journey at www.voyages-sncf.com as there is a great deal of construction and upgrade work taking place. Unlike many countries, this work takes place during 'Office Hours' on weekdays and sometimes trains can be cancelled and replaced by buses.
I hope that helps.