Beyond their fundamental interest in exploring bosonic Dirac physics, Dirac magnons hold potential for the development of efficient spin-transport devices. Here, we demonstrate the experimental realization of gapped terahertz Dirac magnons in an epitaxial Co bilayer on Ru(0001) using spin-polarized electron spectroscopy. Supported by first-principles calculations, we show that although the substrate breaks sublattice symmetry, thereby opening a gap at the $\bar{K}$-points, the local valley topology is preserved. This is evidenced by a spectral weight redistribution, valley-centered curvature concentrations, and a nonzero local Berry phase. Our results establish the interface as a tuning parameter, paving the way for engineering Dirac magnons in a variety of simple ultrathin bilayer magnets.