Irun to Pasajes is a stiff climb-- give yourself the entire day and take it slowly. As well, take snacks as there is nothing until Pasajes. The first steps out of Pasajes once you cross the fjord involves an impressive staircase-like climb to the heights. Again, take your time. Most small ports were built on inlets or natural harbours, which means that one descends from the plateau on arrival, and then you ascend to the plateau again in the morning.
I have walked the del Norte in September (not August) and found that there is a moderate number of pilgrims, but in 2023 much more than in 2022. Albergue accommodation is limited and so you may have to be prepared to take a pension room from time to time. However, the pilgrim vibe is very good-- many are repeaters and there is a greater proportion of Europeans than North Americans than on the Francese.