We say that a vertex or edge colouring of a graph is distinguishing if the only automorphism that preserves this colouring is the identity. A (proper) distinguishing colouring is irreducible if there is no possibility of merging two non-empty classes of colours to obtain a (proper) distinguishing colouring. We show that every graph has an irreducible (proper) distinguishing vertex colouring and that every graph without isolated edge and with at most one isolated vertex has an irreducible (proper) distinguishing edge colouring. Moreover, we show that the existence of any of these colourings for every connected graph (not isomorphic to $K_2$) is equivalent to the Axiom of Choice.