  Single crystals of a honeycomb lattice antiferromagnet, Tb$_2$Ir$_3$Ga$_9$
were synthesized, and the physical properties have been studied. From
magnetometry, a long-range antiferromagnetic ordering at $\approx$12.5 K with
highly anisotropic magnetic behavior was found. Neutron powder diffraction
confirms that the Tb spins lie along the $\va $-axis, parallel to the shortest
Tb-Tb contact. Two field-induced spin-flip transitions are observed when the
field is applied parallel to this axis, separated by a plateau corresponding
roughly to M$\approx$M$_{\rm{s}}$/2. Transport measurements show the
resistivity to be metallic with a discontinuity at the onset of N\'eel order.
Heat capacity shows a $\lambda$-like transition confirming the bulk nature of
the magnetism. We propose a phenomenological spin-Hamiltonian that describes
the magnetization plateau as a result of strong Ising character arising from a
quasidoublet ground state of the Tb$^{3+}$ ion in a site of \textit{C$_s$}
symmetry and expressing a significant bond dependent anisotropy.
