Hi, Helen,
There is a bus that leaves T4 of Barajas at 9:45, which you could likely make, but it gets you to Irun at 3 in the morning. The alternative of going into Madrid, spending a night, and then getting a train or bus is more of a hassle but may be preferable. If I were to arrive in Irun at 3am, I would be sure to go to a hotel with 24 hours reception. I would guess there will be cabs waiting at the bus station. But I’m not positive.
If you’re walking the Norte, you would probably not want to stay in Pamplona. There are buses from Pamplona to Irun but that seems unnecessarily complicated to me.
I think the Norte is justas safe as the Francés. I have walked alone several times.
Middle August is high tourist time on the Norte, so pilgrims will be competing with tourists for beds (except in the albergues de peregrinos of course). Prices in those private places will also be higher than in off-tourist season, when the business people are quite happy to reduce prices for peregrinos.
But the albergue system on the Norte was good. I was there a couple of years ago and was surprised at the growth in private albergues.