It is possible to get the 6pm bus to Pamplona from the bus station in Bilbao, after the Dublin flight that is due in to airport around 4.15. The bus from the airport is easy to catch, as tpmchugh says. I think the price is 1.75 but I forget, really. I got my last ticket online as I had chosen to travel on one of the worst days of the year - the Friday of San Fermin. Actually, given that you have to get in behind the building facing the buses, to get to the desk to buy your ticket, even though you have to pay about three euro to the company rome2rio suggests, it is worth doing it so you just have to show up at the bus rank and keep your eyes peeled for the bus to Pamplona. Seats are assigned, so no hassle. Then you can sleep in Pamplona and head off early in the morning to climb el Perdón. An alternative to staying over in Bilbao. You have choices to make!