  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.
