Previous work on ion irradiation control of FePt thin film magnetic anisotropy is extended to ultrathin films (2–10 nm). The effects of 30 keV He ion irradiation on the magnetic properties are explored as a function of ion fluence and film thickness. Depending on their growth conditions, the thinnest films exhibit different magnetic properties. Although this affects their final magnetic behaviour, we show that after irradiation at 300 °C the easy magnetization axis may rotate entirely from in-plane to out-of-plane at very low fluences, e.g. 2 × 10¹³ He⁺ cm⁻² on 5 nm thick film. This demonstrates the extreme sensitivity of the magnetic anisotropy to ion-induced local L1₀ ordering. Under these conditions, ultrathin films may exhibit perfectly square hysteresis loops with 100% remanent magnetization and low coercivity.