  Topological magnon insulators constitute a growing field of research for
their potential use as information carriers without heat dissipation. We report
an experimental and theoretical study of the magnetic ground-state and
excitations in the van der Waals two-dimensional honeycomb magnet ErBr3. We
show that the magnetic properties of this compound are entirely governed by the
dipolar interactions which generate a continuously degenerate non-collinear
ground-state on the honeycomb lattice with spins confined in the plane. We find
that the magnon dispersion exhibits Dirac-like cones when the magnetic moments
in the ground-state are related by time-reversal and inversion symmetries
associated with a Berry phase \pi as in single-layer graphene. A magnon band
gap opens when the dipoles are rotated away from this state, entailing a finite
Berry curvature in the vicinity of the K and K' Dirac points. Our results
illustrate that the spin-wave dispersion of dipoles on the honeycomb lattice
can be reversibly controlled from a magnetic phase with Dirac cones to a
topological antiferromagnetic insulator with non-trivial valley Chern number.
