Ultra high-energy (UHE) cosmic rays (CRs) from distant sources interact with intergalactic radiation fields, leading to their spallation and attenuation through photo-hadronic processes. At energies of 10^18-10^20 eV, the Universe is practically ‘opaque’ to UHE CRs over several tens of Mpc, thus setting the boundary of the so-called Greisen Zatsepin-Kuz’min (GZK) horizon. Despite this, the residual flux originating from large populations of sources located beyond the GZK horizon can still make a substantial contribution to the UHE CRs observed on Earth, leading to the natural emergence of a strong isotropic component in the UHE CR background. The CRs forming this background are also deflected by intergalactic magnetic fields (IGMFs), in particular those associated with Mpc-scale structures.  These deflections alter the cumulative cooling and interactions of a CR ensemble to modify their spectral shape and composition observed on Earth. In this talk, I will demonstrate that IGMFs can have a marked effect on observed UHE CRs, and that source population models are degenerate with IGMF properties. Interpretation of observations, including the endorsement or rejection of any particular UHE CR source classes, thus needs careful consideration of the structural properties and evolution of IGMFs. Future observations providing tighter constraints on IGMF properties will significantly improve confidence in assessing UHE CR sources and their intrinsic CR production properties.