  We investigate the magnetic properties of the monoclinic D-type
$\rm{Er_2Si_2O_7}$ with a distorted honeycomb lattice using powder and single
crystal neutron scattering techniques, as well as single crystal magnetisation
measurements. The powder neutron diffraction shows that below the ordering
temperature, $T_{\rm N}=1.85$ K, the compound forms a ${\bf q}=0$
antiferromagnetic structure with four sublattices. For $H \! \parallel \! a$,
magnetisation measurements reveal a narrow, but clearly visible plateau at one
third of the magnetisation saturation value. The plateau's stabilisation is
accompanied by a significant increase of the magnetic unit cell, as the
magnetic peaks with fractional indices are observed in single crystal neutron
diffraction experiments. At low-temperatures, the inelastic neutron scattering
measurements reveal the presence of low-energy dispersionless excitations.
Their spectrum is sensitive to the applied field, it significantly softens on
the magnetisation plateau, and demonstrates the behaviour expected for a
non-collinear Ising antiferromagnet away from the plateau.
