  We report on the spin structure of the pyrochlore iridate Nd$_2$Ir$_2$O$_7$
that could be directly determined by means of powder neutron diffraction. Our
magnetic structure refinement unravels a so-called all-in/all-out magnetic
structure that appears in both, the Nd and the Ir sublattice. The ordered
magnetic moments at 1.8 K amount to 0.34(1) $\mu_\mathrm{B}$/Ir$^{4+}$ and
1.27(1) $\mu_\mathrm{B}$/Nd$^{3+}$. The Nd$^{3+}$ moment size at 1.8 K is
smaller than that expected for the Nd$^{3+}$ ground state doublet. On the other
hand, the size of the ordered moments of the Ir$^{4+}$ ions at 1.8 K agrees
very well with the value expected for a $J_\mathrm{eff}$ = 1/2 state based on
the presence of strong spin-orbit coupling in this system. Finally, our
measurements reveal a parallel alignment of the Nd$^{3+}$ moments with the net
moment of its six nearest neighboring Ir$^{4+}$ ions.
