There's no public transport to/from the Piton de la Fournaise.
The main gateway to the volcano is Bourg-Murat. From there, the scenic, zigzagging Route Forestière du Volcan leads to Pas de Bellecombe, about 30km southeast of Bourg-Murat. The gradual change of scenery is mind-boggling. The grassy meadows and cryptomeria forests typical of the Hautes Plaines progressively change to scrubland and Martian landscape.