We investigate the magnetic properties of the monoclinic D-type Er₂Si₂O₇ with a distorted honeycomb lattice using powder and single-crystal neutron scattering techniques, as well as single-crystal magnetization measurements. The powder neutron diffraction shows that below the ordering temperature, TN=1.85K, the compound forms a q=0 antiferromagnetic structure with four sublattices. For H∥a, magnetization measurements reveal a narrow but clearly visible plateau at one-third of the magnetization saturation value. The plateau's stabilization 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, almost dispersionless excitations. Their spectrum is sensitive to the applied field, it significantly softens on the magnetization plateau, and demonstrates the behavior expected for a noncollinear Ising antiferromagnet away from the plateau.