Rome2rio is a perfectly good website, on Apple, Android or on a PC or laptop. I've never used their app (if they have one).
Just Google "Rome2Rio" then enter "Bilbao airport" and " Irun" in the boxes it shows you. It will present various alternative modes or combinations of modes of transport.
It works with most pairs of place names from anywhere in the developed world. Perhaps try it with a journey you're familiar with first, so you get used to the format.
From Bilbao airport to Irun, I would definitely catch the regional bus. The airport is relatively small and the main info desk will point you to where to catch the bus. It takes around two hours and costs less than €15.
For context, it will take you between 5 and seven days to walk along the Camino del Norte, from Irun back to Bilbao!
Buen Camino!