Indeed, not only the so called "San Sebastian" airport is basically in Irun, actually lies on the border of the towns of Irun and Hondarribia, but also it is very close to the Camino - 15 minutes by walking, if you know where to go - using a map or GPS. Actually on my next Camino del Norte, I'm planning to walk directly from the airport (that's why I purchased a credencial from the forum shop).
As for the "alpinista route" between Irun and San Sebastian, all I can say is, you don't know guys what you have missed. How the views from Monte Igueldo (altitude 170m) compare to the view from Jaizkibel on the alpinista route (altitude 547m)? Many people compare the high route over Jaizkibel to the Napoleon Route on the
Camino Frances, saying it is the most beautiful stretch of the Norte, and equally breathtaking - figuratively and literally!
Having said that, I will certaintly use your advice about taking a funicular to Monte Igueldo while I am in San Sebastian
Buen Camino!
...and try to take the Flysch route between Zumaia and Deba (GR-121, red and white markings), otherwise you may miss something spectacular:
https://upload.wikimedia.org/wikipedia/commons/4/46/Sakoneta,_Deba.jpg