  M-theory compactified on a $G_2$ manifold with resolved $E_8$ singularity is
a promising candidate for a unified theory. The experimentally observed masses
of quarks and charged leptons put a restriction on the moduli of the $G_2$
manifold. These moduli in turn uniquely determine the Dirac interactions of the
neutrinos. In the paper, we explicitly compute the Dirac terms for neutrino
mass matrix using the moduli from a localized model with resolved $E_8$
singularities on a $G_2$ manifold. This is a novel approach as the Dirac terms
are not assumed but derived from the structure of quarks' and charged leptons'
masses. Using known mass splittings and mixing angles of neutrinos, we show the
acceptable region for Majorana terms. We also analyse the theoretical region
for Majorana terms induced from the expectation values of right handed
neutrinos through the Kolda-Martin mechanism. The intersection of the two
regions indicates a restriction on neutrino masses. In particular, the lightest
neutrino must have small but non-zero mass. Moreover, this also puts
constraints on possible Majorana contributions from K\"ahler potential and
superpotential, which can be traced down to a restriction on the geometry.We
conclude that the masses of the two heavier light neutrinos are about $0.05
\text{ eV}$ and $0.009 \text{ eV}$ ($0.05 \text{ eV} $ and $0.05 \text{ eV} $))
for normal (inverted) hierarchy. In both hierarchies, we predict the light
neutrinos are mostly Dirac type. Hence neutrino-less double-beta decay will be
small. This is a testable result in a near future. Some bounds on heavy
neutrinos are also derived.
