Magnetization and specific heat measurements on a UIrSi 3 single crystal reveal Ising-like antiferromagnetism below T N = 41.7 K with easy magnetization direction along the c-axis of tetragonal structure. The antiferromagentic ordering is suppressed by magnetic fields > H c (µ 0 H c = 7.3 T at 2 K) applied along the c-axis. The first-order metamagnetic transition at H c exhibits asymmetric hysteresis reflecting a slow reentry of the complex ground-state antiferromagnetic structure with decreasing field. The hysteresis narrows with increasing temperature and vanishes at 28 K. A second-order metamagnetic transition is observed at higher temperatures. The point of change of the order of transition in the established H-T magnetic phase diagram is considered as the tricritical point (at T tc = 28 K and µ 0 H tc = 5.8 T). The modified-Curie-Weiss-law fits of temperature dependence of the a-and c-axis susceptibility provide opposite signs of Weiss temperatures,  p a ~ -51 K and  p c ~ +38 K, respectively. This result and the small value of µ 0 H c contrasting to the high T N indicate competing ferromagnetic and antiferromagnetic interactions responsible for the complex antiferromagnetic ground state. The simultaneous electronic-structure calculations focused on the total energy of ferromagentic and various antiferromagnetic states, the U magnetic moment and magnetocrystalline anisotropy provide results consistent with experimental findings and the suggested physical picture of the system.

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