Hello James, I tried to walk out to Irun last year, but for a number of reasons wasn't terribly successful. I did walk out to St Etienne de Baigarroy Stayed overnight at a gite there) and intended to hit the GR10 from that point (it seems that most people walk it from Irun rather than the other way though). When I went to the tourist office there however, I was advised not to attempt it the next day as the weather forecast was bad. With a choice of sitting around that lovely village for an extra day, or catching a train (turned out to be a bus that stopped at all the train stations!) to Bayonne, I opted for the train. Although I was dissapointed that I missed the mountain path I was very glad I went this way. The path from Bayonne is very well marked and during peak pilgrim time there is someone in attendance at a desk in the Cathedral - I think from about 10 - 1.00 - who answers questions, not only for the pilgrims (of which there were only 2 that day) but for other people who might be interested.
The Amis people at the Cathedral have a list of accomodation options in Bayonne, and to the border - although it is sparse. Be warned St Jean de Luz and its neighbour across the river Ciboure have very little accomodation options in peak tourist time, especially on the weekends - make sure you book. I arrived at 6.00pm and there was NOTHING (other than 5 star!) available. I went to a Basque mass and speaking to the locals they all suggested that I go and sleep on the beach. I wasn't very keen on that idea, and so I just found (in the dark) a place that was in shadow under a tree and off the road and slept on the grass - or tried to as there was a party in town and the music was very loud!
It is a 2 day walk from Bayonne to Irun and so I did it in three days - SJPDP to St Etienne, Bayonne to St Jen de Luz, St Jean to Irun. Hope this helps, Janet