Magnetization and specific heat measurements on a UIrSi3 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$_c$ ({mu}$_0$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 {mu}$_0$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, {Theta}$_p^a$ ~ -51 K and {Theta}$_p^c$ ~ +38 K, respectively. This result and the small value of {mu}$_0$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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