Is the festival Beltane named for an Irish god Bel?
Short answer: probably not.
The Keltic peoples of the Continent knew of a god Belenos (attested in various spellings) who, during the Roman period, was identified with Apollo.
Belenos clearly = *bel-, “shining, bright” + infixed -n-, (denotes lordship, mastery, or preeminence) + -os, (masculine singular ending). The “mastery infix,” interestingly, features in the names of a number of Keltic deities: among them Cernunnos, “Horned Lord” or “Preeminently Horned” and Epona, “Lady Horse” or “Preeminent Horse.” So Belenos is “Bright Lord” or “the Preeminently Bright.”
Did the Keltic-speaking peoples of Britain know such a god?
If so, the evidence is minimal, and there's none whatsoever that the Irish knew him. ('Beltane' is an Irish word in origin.) We cannot assume that the Insular Kelts worshiped every god that their Continental kin did.
So alas, Beltane is probably not “Bel's fire.”