By logging encounters between planetesimals and planets, we compute the distribution of encounters in a numerically integrated two-planet system that is migrating due to interactions with an exterior planetesimal belt. Capture of an irregular satellite in orbit about a planet through an exchange reaction with a binary planetesimal is only likely when the binary planetesimal undergoes a slow and close encounter with the planet. Taking care to consider where a planet-orbit-crossing binary planetesimal would first be tidally disrupted, we estimate the probability of both tidal disruption and irregular satellite capture. We estimate that the probability that the secondary of a binary planetesimal is captured and becomes an irregular satellite about a Neptune mass outer planet is about 1/100 for binaries with masses and separations similar to transneptunian planetesimal binaries. Scaling functions are given allowing probabilities to be estimated for satellite capture from binary planetesimals with other masses, mass ratios and separations. This capture probability is only weakly dependent upon the binary's tidal disruption radius. We find that the probability that a satellite is captured by the inner planet is one to two orders of magnitude lower than by the outer planet. If young exoplanetary debris discs contain a population of binary planetesimals, then an outwards migrating outer planet should host a recently captured irregular satellite population.