  The magnetic order of the localized Eu$^{2+}$ spins in optimally-doped
Eu(Fe$_{1-x}$Ir$_{x}$)$_{2}$As$_{2}$ ($\mathit{x}$ = 0.12) with superconducting
transition temperature $\mathit{T_{SC}}$ = 22 K was investigated by
single-crystal neutron diffraction. The Eu$^{2+}$ moments were found to be
ferromagnetically aligned along the $\mathit{c}$-direction with an ordered
moment of 7.0(1) $\mu_{B}$ well below the magnetic phase transition temperature
$\mathit{T_{C}}$ = 17 K. No evidence of the tetragonal-to-orthorhombic
structural phase transition was found in this compound within the experimental
uncertainty, in which the spin-density-wave (SDW) order of the Fe sublattice is
supposed to be completely suppressed and the superconductivity gets fully
developed. The ferromagnetic groud state of the Eu$^{2+}$ spins in
Eu(Fe$_{0.88}$Ir$_{0.12}$)$_{2}$As$_{2}$ was supported by the first-principles
density functional calculation. In addition, comparison of the electronic
structure calculations between Eu(Fe$_{0.875}$Ir$_{0.125}$)$_{2}$As$_{2}$ and
the parent compound EuFe$_{2}$As$_{2}$ indicates stronger hybridization and
more expanded bandwith due to the Ir substitution, which together with the
introduction of electrons might work against the Fe-SDW in favor of the
superconductivity.
