  Ensembles of magnetic skyrmions in confined geometries are shown to exhibit
thermally driven motion on two different time scales. The intrinsic fluctuating
dynamics ($t\sim 1~$ps) is governed by short-range symmetric and antisymmetric
exchange interactions, whereas the long-time limit ($t\gtrsim10\,$ns) is
determined by the coaction of skyrmion-skyrmion-repulsion and the system's
geometry. Micromagnetic simulations for realistic island shapes and sizes are
performed and analyzed, indicating the special importance of skyrmion dynamics
at finite temperatures. We demonstrate how the competition between skyrmion
mobility and observation time directly affects the addressability of skyrmionic
bits, which is a key challenge on the path of developing skyrmion-based
room-temperature applications. The presented quasiparticle Monte Carlo approach
offers a computationally efficient description of the diffusive motion of
skyrmion ensembles in confined geometries, like racetrack memory setups.
