Hi Dave!
1. You can get your credencial stamped at the albergue in Irun. It opens at 4pm. It's very central, just a few minutes from the train station. Here's some more
info about the albergue. But if this is too late for you, don't worry, you can get your credencial stamped anywhere, even a bar! The tourist information is also a good bet.
2. The albergue in Irun has 60 beds. As you'll be there over an hour before opening time, you should be able to get a bed without a problem (although depending on how busy it is, there might already be a queue when you get there... FYI, as you already seem to know, pilgrims queue up until the hospitalero arrives - you can get times on the Gronze site. Sometimes you need to count the backpacks and compare to the number of beds, to judge whether it's worth waiting, or whether you need to seek alternatives)
3. There is nothing between Irun and Pasajes de San Juan (the path goes along the coastal headland). And this stretch is 17km, starting with a 300m climb... The albergue in Pasajes is first-come-first-served, like all donativos and municipal albergues. As trecile says, it's very unlikely that you'll get a bed, especially as you'd only be arriving around 6-7pm.
So, the long and the short of it is, if you decided to walk straight away, you'd need to book private accommodation, and you'd need to get your stamp somewhere other than the albergue, so that you can hit the trail asap!