  Using resonant magnetic x-ray scattering we address the unresolved nature of
the magnetic groundstate and the low-energy effective Hamiltonian of
Sm$_2$Ir$_2$O$_7$, a prototypical pyrochlore iridate with a finite temperature
metal-insulator transition. Through a combination of elastic and inelastic
measurements, we show that the magnetic ground state is an all-in all-out
(AIAO) antiferromagnet. The magnon dispersion indicates significant electronic
correlations and can be well-described by a minimal Hamiltonian that includes
Heisenberg exchange ($J=27.3(6)$ meV) and Dzyaloshinskii-Moriya interaction
($D=4.9(3)$ meV), which provides a consistent description of the magnetic order
and excitations. In establishing that Sm$_2$Ir$_2$O$_7$ has the requisite
inversion symmetry preserving AIAO magnetic groundstate, our results support
the notion that pyrochlore iridates may host correlated Weyl semimetals.
