We used single-crystal x-ray and neutron diffraction to investigate the crystal and magnetic structures of trigonal lattice iridate Ca2Sr2IrO6. The crystal structure is determined to be $Rbar3$ with two distinct Ir sites. The system exhibits long-range antiferromagnetic order below $T_N = 13.1$ K. The magnetic wave vector is identified as $(0,0.5,1)$ with ferromagnetic coupling along the $a$ axis and antiferromagnetic correlation along the $b$ axis. Spins align dominantly within the basal plane along the [1,2,0] direction and tilt 34$^circ$ towards the $c$ axis. The ordered moment is 0.66(3) $mu_B$/Ir, larger than other iridates where iridium ions form corner- or edge-sharing $rm IrO_6$ octahedral networks. The tilting angle is reduced to $approx19^circ$ when a magnetic field of 4.9 Tesla is applied along the $c$ axis. Density functional theory calculations confirm that the experimentally determined magnetic configuration is the most probable ground state with an insulating gap $sim0.5$~eV.
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