  Magnetic skyrmions are topologically-distinct swirls of magnetic moments
which display particle-like behaviour, including the ability to undergo
thermally-driven diffusion. In this paper we study the thermally activated
motion of arrays of skyrmions using temperature dependent micromagnetic
simulations where the skyrmions form spontaneously. In particular, we study the
interaction of skyrmions with grain boundaries, which are a typical feature of
sputtered ultrathin films used in experimental devices. We find the
interactions lead to two distinct regimes. For longer lag times the grains lead
to a reduction in the diffusion coefficient, which is strongest for grain sizes
similar to the skyrmion diameter. At shorter lag times the presence of grains
enhances the effective diffusion coefficient due to the gyrotropic motion of
the skyrmions induced by their interactions with grain boundaries. For grain
sizes significantly larger than the skyrmion diameter clustering of the
skyrmions occurs in grains with lower magnetic anisotropy.
