Single-crystal neutron diffraction, inelastic neutron scattering, bulk magnetization measurements, and first-principles calculations are used to investigate the magnetic properties of the honeycomb lattice Tb2Ir3Ga9. While the Rln2 magnetic contribution to the low-temperature entropy indicates a Jeff=1/2 moment for the lowest-energy crystal-field doublet, the Tb3+ ions form a canted antiferromagnetic structure below 12.5 K. Due to the Dzyaloshinskii-Moriya interactions, the Tb moments in the ab plane are slightly canted towards b with a canted moment of 1.22 μB per formula unit. A minimal xxz spin Hamiltonian is used to simultaneously fit the spin-wave frequencies along the high-symmetry directions and the field dependence of the magnetization along the three crystallographic axes. Long-range magnetic interactions for both in-plane and out-of-plane couplings up to the second nearest neighbors are needed to account for the observed static and dynamic properties. The z component of the exchange interactions between Tb moments is larger than the x and y components. This compound also exhibits bond-dependent exchange with negligible nearest-neighbor exchange coupling between moments parallel and perpendicular to the 4f orbitals. Despite the Jeff=1/2 moments, the spin Hamiltonian is denominated by a large in-plane anisotropy Kz∼−1meV. DFT calculations confirm the antiferromagnetic ground state and the substantial interplane coupling at larger Tb-Tb distances.