  The discovery of massless Dirac electrons in graphene and topological
Dirac-Weyl materials has prompted a broad search for bosonic analogues of such
Dirac particles. Recent experiments have found evidence for Dirac magnons above
an Ising-like ferromagnetic ground state in a two-dimensional (2D) kagome
lattice magnet and in the van der Waals layered honeycomb crystal CrI$_3$, and
in a 3D Heisenberg magnet Cu$_3$TeO$_6$. Here we report on our inelastic
neutron scattering investigation on large single crystals of a stacked
honeycomb lattice magnet CoTiO$_3$, which is part of a broad family of ilmenite
materials. The magnetically ordered ground state of CoTiO$_3$ features
ferromagnetic layers of Co$^{2+}$, stacked antiferromagnetically along the
$c$-axis. We discover that the magnon dispersion relation exhibits strong
easy-plane exchange anisotropy and hosts a clear gapless Dirac cone along the
edge of the 3D Brillouin zone. Our results establish CoTiO$_3$ as a model
pseudospin-$1/2$ material to study interacting Dirac bosons in a 3D quantum XY
magnet.
