  Massless Dirac fermions occur as low-energy modes in several
quasi-two-dimensional condensed matter systems such as graphene, the surface of
bulk topological insulators, and in layered organic semiconductors. When the
rotational symmetry in such systems is reduced either by an in-plane electric
field or an intrinsic tilt of the Dirac cones, the allowed dipolar optical
transitions evolve from a few selected transitions into a wide fan of interband
transitions. We show that the Lorentz covariance of the low-energy carriers
allows for a concise analysis of the emerging magneto-optical properties. We
predict that infrared absorption spectra yield quantitative information on the
tilted Dirac cone structure in organic compounds such as
$\alpha$-(BEDT-TTF)$_2$I$_3$.
