Many tetragonal compounds LnIr2Si2 (Ln = lanthanoid) occur in two polymorphous phases and are therefore well suited to study the relationship between crystal structure and magnetic properties. We have grown GdIr2Si2 single crystals of both polymorphs from a high-temperature indium flux and investigated their anisotropic magnetic properties. The higher symmetric form with the ThCr2Si2 structure (space group I4/mmm) orders antiferromagnetically at TN=87K while for the lower symmetric compound in the CaBe2Ge2 structure (space group P4/nmm) we determined a much lower Néel temperature TN=12K. Our magnetic characterization of the single crystals reveals that for both compounds the magnetic moments are aligned in the a−a plane of the tetragonal lattice, but that the change of the symmetry strongly effects the inplane alignment of the moment orientation. For Ln=Er and Ho, we confirmed the existence of LnIr2Si2 in the space group P4/nmm. The magnetic properties of these lower symmetric compounds are in remarkable difference to their related compounds in the space group I4/mmm.