  Filling-enforced Dirac semimetals, or those required at specific fillings by
the combination of crystalline and time-reversal symmetries, have been proposed
and discovered in numerous materials. However, Dirac points in these materials
are not generally robust against breaking or modifying time-reversal symmetry.
We present a new class of two-dimensional Dirac semimetal protected by the
combination of crystal symmetries and a special, antiferromagnetic
time-reversal symmetry. Systems in this class of magnetic layer groups, while
having broken time-reversal symmetry, still respect the operation of
time-reversal followed by a half-lattice translation. In contrast to 2D
time-reversal-symmetric Dirac semimetal phases, this magnetic Dirac phase is
capable of hosting just a single isolated Dirac point at the Fermi level, and
that Dirac point can be stabilized solely by symmorphic crystal symmetries. We
find that this Dirac point represents a new quantum critical point, and lives
at the boundary between Chern insulating, antiferromagnetic topological
crystalline insulating, and trivial insulating phases. We present density
functional theoretic calculations which demonstrate the presence of this 2D
magnetic Dirac semimetallic phase in FeSe monolayers and discuss the
implications for engineering quantum phase transitions in these materials.
