You might actually find it easier to take the train, because there are not too many flights from CDG to Biarritz, and once you get to Birarritz airpoort, youāre then going to have to take a bus into Bayonne or Biarritz to pick up the train, which actually goes to Hendaye rather than Irun. (The two towns are right across the river from one another, and itās only a few hundred meters from the Hendaye train station to the bridge into Irun).
Check the train schedules at
https://en.oui.sncf/en/. If you donāt find a good connection from the airport, then check the schedule from Gare Montparnasse in Paris. From Gare Montparnasse, there are many trains during the day, and itās easy to get there from the airport by train or bus.
Overall, it will probably be less time by train, since you donāt have to make as many connections. Far fewer hassles, too, with security, etc., and you can stretch your legs out.
In Irun, itself, we actually stayed at a Casa Rurale called Postigu. Itās outside Irun, and cuts out about 3-4 kilometers off the first dayās walk to San Sebastian. Itās quite lovely, but not cheap.