I plan the same transfer, did not find good routes online, therefore drew my own. I don't expect to go via St. Palais but stop at the gite in Larribar-Sorhapuru (near the Beneditenia 3-Caminos stone). From there it's about 82 km to Irun. My first stop will be in Hélette, and then probably in Espelette and Ascain, so it should be an easy four days. Ascain apparently does not have (cheap) pilgrim accommodation; on French websites a different, slightly longer route is therefore recommended between Espelette and Irun. My walk from Beneditenia to Irun will be on small country roads.
If you are interested, I can e-mail you the maps.