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Optical properties and lattice dynamics of hexagonal M₂Mo₃O₈ (M = Fe, Mn) single crystals are studied in a wide temperature range by means of infrared (IR) reflectivity and Raman scattering at the temperatures above and below the magnetic transitions. Experimental data for optical phonons are compared to group theoretical mode analysis and complimentary DFT and DMFT lattice dynamics calculations. At temperatures below T_N(Fe)=60 K several additional IR- and Raman-active modes are detected in experimental spectra of Fe₂Mo₃O₈. We also found several IR-active modes in the 2500 – 3500 cm^-1 range in Fe₂Mo₃O₈ at low temperatures. We attribute them to optical transitions from the ground state to the split levels of the ⁵T₂ state of Fe²⁺ ions in the tetrahedral coordination.