  Oxides containing iridium ions display a range of magnetic and conducting
properties that depend on the delicate balance between interactions and are
controlled, at least in part, by the details of the crystal architecture. We
have used muon-spin rotation ($\mu$SR) to study the local field in four iridium
oxides, Ca$_4$IrO$_6$, Ca$_5$Ir$_3$O$_{12}$, Sr$_3$Ir$_2$O$_7$ and
Sr$_2$IrO$_4$, which show contrasting behavior. Our $\mu$SR data on
Ca$_4$IrO$_6$ and Ca$_5$Ir$_3$O$_{12}$ are consistent with conventional
antiferromagnetism where quasistatic magnetic order develops below $T_{\rm
N}=13.85(6)$ K and 7.84(7) K respectively. A lower internal field is observed
for Ca$_5$Ir$_3$O$_{12}$, as compared to Ca$_4$IrO$_6$ reflecting the presence
of both Ir$^{4+}$ and Ir$^{5+}$ ions, resulting in a more magnetically dilute
structure. Muon precession is only observed over a restricted range of
temperature in Sr$_3$Ir$_2$O$_7$, while the Mott insulator Sr$_2$IrO$_4$
displays more complex behavior, with the $\mu$SR signal containing a single,
well-resolved precession signal below $T_{\rm N}=230$\,K, which splits into two
precession signals at low temperature following a reorientation of the spins in
the ordered state.
