The effects of ion irradiation in materials for research are usually limited to a shallow surface layer of the order of one micrometre in depth. Determining the mechanical properties of such irradiated materials requires techniques with high spatial resolution. Nanoindentation is a relatively simple method for investigating these shallow layers with the advantage that statistically rich data sets for elastic and plastic property values can be generated. However, interpretation of the results requires and understanding of the material response, including the extent of the plastic zone with respect to the irradiated layer, pile-up or sink-in of material around the indentation site that affect the calculated contact area and hence derived mechanical property values. An Fe+ self-irradiated Fe12%Cr alloy was investigated with three different indenter tip geometries, a Cube corner, Berkovich and 10 micron radius indenter. Sharp indenters provide sufficiently small plastic zones to be contained within the irradiated layer but pop-in events and pile-up need to be taken into account for correct interpretation of the mechanical properties of the irradiated material.