We report on the effects of grain size in antiferromagnetic IrMn layers exchange bias to Co2FeSi. We also report on an enhanced effect where Mn layers are inserted in the interface. The exchange-biased IrMn/Co2FeSi samples were grown by a HiTUS system, which allows us to control the grain size. The smaller IrMn grains were too small to give a large Hex while an Mn layer 0.5 nm thick dramatically increased Hex. This significant increase is attributed to optimization of the Mn concentration at the interface. This grain-size and interface tuning offers a way to control the exchange bias in such systems.