It is well-known that FeRh undergoes an antiferromagnetic to ferromagnetic (FM) phase transition where the high temperature phase is a low coercivity FM material. However, little is known about the effect of lateral confinement on the transition dynamics in FeRh thin films. Here, we pattern FeRh thin films into arrays of nanowires with a large aspect ratio (100:1) and, with ultrafast probing of the magnetic state in an applied magnetic field, we determine the influence of demagnetization fields on the stability of laser induced FM domains. In particular, with pump-probe Kerr measurements, we demonstrate that, when a magnetic field is applied along the nanowire length, the nanowire arrays exhibit an FM phase (>3.0ns) that is longer-lived than that observed for continuous thin films (≈2.0 ns). With electrical measurements, we also show that the transition temperature depends on the relative orientation of the magnetic field. Indeed, when the FeRh film is patterned with sub-μm features, the transition temperature decreases by up to 7 K depending on the field direction at applied magnetic fields of 1 T. The effects of sample heating are explored using finite-element simulations to determine the heat dissipation following laser excitation across a range of FeRh nanowire widths. These simulations confirm that the increased lifetimes of the magnetic-field-aligned FM domains in the nanowire arrays are not due to differences in heat dissipation. This suggests that FM domain growth and relaxation through the ultrafast phase transition in FeRh nanowires is strongly dependent on the shape anisotropy. This knowledge is important for the fine control of the phase transition in patterned FeRh thin films for nanoscale devices.