I caught the bus from Bilboa to Irun in June. There was no trouble buying at the bus station on the day. There was, however, an enormously long queue to purchase tickets from the window and a small one to purchase from a machine. I took a punt and joined the short one. The fellow behind me very graciously assisted me to purchase the ticket which was just as well as, left to my own devices, I was going to take a while.
By the way, are you actually wanting to go to Irun? I was heading back to Paris so wanted to take the bus to Hendaye. The woman in the information booth told me that the bus would say Irun, the ticket would say Irun but the bus would actually go as far as Hendaye. I was a little sceptical and very hesitantly said "Hendaye" when the bus driver asked me where I was going but he confirmed that he was in fact going there.
The ALSA website only accepts cards that originate in Spain but it does take Paypal and I have successfully bought a ticket online using that service.
