  A key challenge in materials discovery is to find high-temperature
superconductors. Hydrogen and hydride materials have long been considered
promising materials displaying conventional phonon-mediated superconductivity.
However, the high pressures required to stabilize these materials have
restricted their application. Here, we present results from high-throughput
computation, considering a wide range of high-symmetry ternary hydrides from
across the periodic table at ambient pressure. This large composition space is
then reduced by considering thermodynamic, dynamic, and magnetic stability,
before direct estimations of the superconducting critical temperature. This
approach has revealed a metastable ambient-pressure hydride superconductor,
Mg$_2$IrH$_6$, with a predicted critical temperature of 160 K, comparable to
the highest temperature superconducting cuprates. We propose a synthesis route
via a structurally related insulator, Mg$_2$IrH$_7$, which is thermodynamically
stable above 15 GPa and discuss the potential challenges in doing so.
