One of the many ideas of the origin of the name of Beltane is that it comes from the Celtic God Belenos. Belenos was one of the most widespread Celtic Gods of the Ancient World. Known from Italy in the East into Gaul and as far as Ireland in the West.
Beltane fires were lit to encourage the sun's warmth. These fires also had restorative properties and cattle were herded between them before being loosed on the new spring pastures. From this it is likely that Belenos was a fire deity, a patron of flame and the sun's restorative powers (which explains his classical association with Apollo).
Belenos, along with Apollo, was also associated as a God of Healing. Which is definitely fitting for this time of the year, since the Earth is in need of healing after the harshness of Winter.
Some historians believe that Belenos was a different aspect of Bile who was married to the Irish Mother Goddess, Danu. This fits with the aspect of the God as Lover during this Sabbat.
It is my personal opinion that Belenos is indeed the root of the Sabbat Beltane and that the fires were most likely lit in his honor to aid energy to Sun (Apollo-Belenos) so that he could produce a good offspring from his union with the Goddess, Mother Earth.