Single crystals of a honeycomb lattice antiferromagnet, Tb2Ir3Ga9, were synthesized, and the physical properties have been studied. From magnetometry, a long-range antiferromagnetic ordering at ≈12.5 K with highly anisotropic magnetic behavior was found. Neutron powder diffraction confirms that the Tb spins lie along the a-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≈Ms/2. Transport measurements show the resistivity to be metallic with a discontinuity at the onset of Néel order. Heat capacity shows a λ-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 Tb3+ ion in a site of Cs symmetry and expressing a significant bond-dependent anisotropy.