How to get to St. Hilaire-de-Riez from La Rochelle Airport. Ryanair don't have tranfer partners. Keycamp transfers are almost the cost of the accommodation!
I think you may have a problem getting there with public transport. You would have to take the train from La Rochelle to Nantes, and then another to SHDR, which is a very roundabout route and would take all day.
Taking into account the cost of taxis at either end, you might be better off, and certainly get there more quickly, by hiring a car.