  In the pyrochlore iridates, \textit{R}$_{2}$Ir$_{2}$O$_{7}$ (\textit{R} =
Lanthanide, Y), determination of the magnetic structure of the iridium moments
remains an outstanding problem despite the role this is expected to play in the
formation of novel band structures and topologies in these materials. In this
work, a new analysis of the experimentally measured spontaneous muon spin
precession frequency is presented which incorporates both probabilistic and
\textit{ab-initio} modeling techniques to determine the ground state magnetic
structure. It is shown that the experimentally observed results are consistent
only with a magnetically ordered Ir$^{4+}$ sublattice with the so-called
'all-in/all-out' magnetic structure, and that the electronic state of the
Ir$^{4+}$ is best described by the $J_{eff} = \frac{1}{2}$ model in several
member compounds. Through this approach it is also demonstrated that such a
simple structure is not likely to be present on the rare-earth sublattice which
contain much larger localized moments.
